Battery, negative electrode active material, and electric tool

ABSTRACT

A battery is provided including a positive electrode; a negative electrode including a first negative electrode active material; and an electrolytic solution, wherein the first negative electrode active material includes a core portion having a core portion surface, wherein the core portion has a median diameter of 0.3 μm to 20 μm, and a covering portion that covers at least part of the core portion surface, wherein the covering portion comprises at least Si, O and at least of one element M1 selected from Li, carbon (C), Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na, and K.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/526,133, filed on Jun. 18, 2012, now U.S. Pat. No.9,048,485, which claims priority to Japanese Priority Patent ApplicationJP 2011-141005 filed in the Japan Patent Office on Jun. 24, 2011, theentire content of which is hereby incorporated by reference.

BACKGROUND

The present application relates to a negative electrode for alithium-ion secondary battery, the negative electrode containing anegative-electrode active material capable of occluding and releasinglithium ions, a lithium-ion secondary battery including the negativeelectrode, a battery pack, an electric vehicle, a power storage system,an electric tool, and an electronic device, which include the secondarybattery.

In recent years, electronic devices typified by, for example, cellularphones and personal digital assistants (PDAs) have been widely used.Further size and weight reduction and longer life of electronic deviceshave been strongly demanded. Thus, there have been advances in thedevelopment of batteries serving as power sources, in particular, smalland lightweight secondary batteries having a high energy density.Recently, various applications of secondary batteries to, for example,battery packs, electric vehicles, such as electric automobiles, powerstorage systems, such as power servers for household use, and electrictools, such as electric drills, as well as electronic devices describedabove have been studied.

Secondary batteries using various charge-discharge principles have beenreported. In particular, lithium-ion secondary batteries that utilizethe occlusion and release of lithium ions hold great promise becausethey have higher energy densities than lead-acid batteries,nickel-cadmium batteries, and other batteries.

A lithium-ion secondary battery includes a positive electrode, anegative electrode, and an electrolytic solution. The negative electrodecontains a negative-electrode active material capable of occluding andreleasing lithium ions. As the negative-electrode active material, acarbon material, such as graphite, is widely used. Recently, secondarybatteries have been required to have higher battery capacities. Thus,the use of Si has been studied. The theoretical capacity of Si (4199mAh/g) is much higher than the theoretical capacity of graphite (372mAh/g), so the battery capacity should be significantly improved.

However, the use of Si as a negative-electrode active material causesextreme expansion and contraction of the negative-electrode activematerial during charge and discharge, so that the negative-electrodeactive material is liable to be cracked mainly in the vicinity of itssurface. When the negative-electrode active material is cracked, ahigh-reactive newly-formed surface (an active surface) is formed,thereby increasing the surface area (reactive area) of thenegative-electrode active material. As a result, the decompositionreaction of an electrolytic solution occurs on the newly-formed surface.The electrolytic solution is consumed to form a coating film derivedfrom the electrolytic solution on the newly-formed surface. Thus,battery characteristics, such as cycle characteristics, are liable todecrease.

Thus, in order to improve battery characteristics, such as cyclecharacteristics, various configurations of the lithium-ion secondarybatteries have been studied.

Specifically, to improve cycle characteristics and safety, Si andamorphous SiO₂ are simultaneously deposited by a sputtering method (forexample, see Japanese Unexamined Patent Application Publication No.2001-185127). To obtain excellent battery capacity and safetyperformance, electron-conductive material layers (carbon material) arearranged on surfaces of SiO_(x) particles (for example, see JapaneseUnexamined Patent Application Publication No. 2002-042806). To improvehigh rate charge-discharge characteristics and cycle characteristics, anegative-electrode active material layer containing Si and O is formedin such a manner that the oxygen content is increased with decreasingdistance from a negative-electrode collector (for example, see JapaneseUnexamined Patent Application Publication No. 2006-164954). To improvecycle characteristics, a negative-electrode active material layercontaining Si and O is formed in such a manner that the average oxygencontent in the whole negative-electrode active material layer is 40atomic percent or less and that the average oxygen content is increasedwith decreasing distance from a negative-electrode collector (forexample, see Japanese Unexamined Patent Application Publication No.2006-114454). In this case, a difference in average oxygen contentbetween a portion near the negative-electrode collector and a portionremote from the negative-electrode collector is in the range of 4 atomicpercent to 30 atomic percent.

To improve initial charge-discharge characteristics and the like, anano-composite including a Si phase, SiO₂, and metal oxide M_(y)O isused (for example, see Japanese Unexamined Patent ApplicationPublication No. 2009-070825). To improve cycle characteristics, powderedSiO_(x) (0.8≦x≦1.5, particle size range: 1 μm to 50 μm) and acarbonaceous material are mixed and fired at 800° C. to 1600° C. for 3hours to 12 hours (for example, see Japanese Unexamined PatentApplication Publication No. 2008-282819). To shorten an initial chargetime, a negative-electrode active material expressed as Li_(a)SiO_(x)(0.5≦a−x≦1.1 and 0.2≦x≦1.2) is used (for example, see InternationalPublication No. WO2007/010922). In this case, Li is deposited byevaporation on an active material precursor containing Si and O. Toimprove charge-discharge cycle characteristics, the composition ofSiO_(x) is controlled in such a manner that the molar ratio of the Ocontent to the Si content of a negative-electrode active material is inthe range of 0.1 to 1.2 and that a difference between the maximum valueand the minimum value of the molar ratio of the O content to the Sicontent in the vicinity of a boundary between the negative-electrodeactive material and a current collector is 0.4 or less (for example, seeJapanese Unexamined Patent Application Publication No. 2008-251369). Toimprove load characteristics, a Li-containing porous metal oxide(Li_(x)SiO: 2.1≦x≦4) is used (for example, Japanese Unexamined PatentApplication Publication No. 2008-177346).

To improve charge-discharge cycle characteristics, a hydrophobic layerof a silane compound, a siloxane compound, or the like is formed on athin film containing Si (for example, see Japanese Unexamined PatentApplication Publication No. 2007-234255). To improve cyclecharacteristics, a conductive powder in which surfaces of SiO_(x)(0.5≦x<1.6) particles are covered with graphite coating films is used(for example, see Japanese Unexamined Patent Application Publication No.2009-212074). In this case, on Raman spectroscopy analysis, eachgraphite coating film develops broad peaks at 1330 cm⁻¹ and 1580 cm⁻¹Raman shift, and an intensity ratio I₁₃₃₀/I₁₅₈₀ is 1.5<I₁₃₃₀/I₁₅₈₀<3. Toimprove a battery capacity and cycle characteristics, a powder including1% by mass to 30% by mass of particles is used, the particles eachhaving a structure in which Si microcrystals (crystal size: 1 nm to 500nm) are dispersed in SiO₂ (for example, see Japanese Unexamined PatentApplication Publication No. 2009-205950). In this case, in a particlesize distribution by a laser diffraction/scattering type particle sizedistribution measurement method, the 90% accumulated diameter (D90) ofthe power is 50 μm or less, and the particle diameters of the particlesare less than 2 μm. To improve cycle characteristics, SiO_(x)(0.3≦x≦1.6) is used, and an electrode unit is pressurized at a pressureof 3 kgf/cm² or more during charge and discharge (for example, seeJapanese Unexamined Patent Application Publication No. 2009-076373). Toimprove overcharge characteristics, over-discharge characteristics, andthe like, an oxide of silicon with a silicon-oxygen atomic ratio of 1:y(0<y<2) is used (for example, see Japanese Patent No. 2997741).

Furthermore, in order to electrochemically accumulate or release a largeamount of lithium ions, an amorphous metal oxide is provided on surfacesof primary particles of Si or the like (for example, see JapaneseUnexamined Patent Application Publication No. 2009-164104). The Gibbsfree energy when the metal oxide is formed by oxidation of a metal islower than the Gibbs free energy when Si or the like is oxidized. Toachieve a high capacity, high efficiency, a high operating voltage, andlong lifetime, it is reported that a negative-electrode material inwhich the oxidation numbers of Si atoms satisfy predeterminedrequirements is used (for example, see Japanese Unexamined PatentApplication Publication No. 2005-183264). The negative-electrodematerial contains Si with an oxidation number of zero, a Si compoundhaving a Si atom with an oxidation number of +4, and a lower oxide of Sihaving a silicon atoms with oxidation numbers of more than zero and lessthan +4.

SUMMARY

Electronic devices and so force have higher performance and morefunctions and are more frequently used. Thus, lithium-ion secondarybatteries tend to be frequently charged and discharged. Hence,lithium-ion secondary batteries are required to have further improvedbattery characteristics.

It is desirable to provide a negative electrode for a lithium-ionsecondary battery, the negative electrode providing excellent batterycharacteristics, a lithium-ion secondary battery, a battery pack, anelectric vehicle, a power storage system, an electric tool, and anelectronic device.

A negative electrode for a lithium-ion secondary battery according to anembodiment of the present application includes an active material, inwhich the active material includes a core portion capable of occludingand releasing lithium ions, and a covering portion arranged on at leastpart of a surface of the core portion, in which the covering portioncontains, as constituent elements, Si, O, and at least one element M1selected from Li, C, Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo,Ag, Sn, Ba, W, Ta, Na, and K, and the atomic ratio y (O/Si) of O to Siis 0.5≦y≦1.8. A lithium-ion secondary battery according to an embodimentof the present application includes a positive electrode, a negativeelectrode, and an electrolytic solution, in which the negative electrodecontains the same active material as that of the foregoing negativeelectrode for a lithium-ion secondary battery. A battery pack, anelectric vehicle, a power storage system, an electric tool, or anelectronic device according to an embodiment of the present applicationincludes the lithium-ion secondary battery according to an embodiment ofthe present application.

In the negative electrode for a lithium-ion secondary battery or thelithium-ion secondary battery according to an embodiment of the presentapplication, the active material of the negative electrode includes thecovering portion on the surface of the core portion, in which thecovering portion contains, as constituent elements, Si, O, and elementM1, such as Li, and the atomic ratio y of O to Si is 0.5≦y≦1.8. It isthus possible to obtain excellent battery characteristics. Furthermore,for the battery pack, the electric vehicle, the power storage system,the electric tool, or the electronic device according to an embodimentof the present application, it is possible to obtain the same effect.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a sectional view illustrating a structure of a negativeelectrode for a lithium-ion secondary battery according to an embodimentof the present application;

FIG. 2 is a photograph illustrating a sectional structure of anegative-electrode active material, the photograph being taken with ascanning electron microscope (SEM);

FIG. 3 is a sectional view illustrating a structure of a lithium-ionsecondary battery (prismatic type) according to an embodiment of thepresent application;

FIG. 4 is a sectional view taken along line IV-IV in FIG. 3 thatillustrates the lithium-ion secondary battery;

FIG. 5 is a schematic plan view illustrating structures of a positiveelectrode and a negative electrode illustrated in FIG. 4;

FIG. 6 is a sectional view illustrating a structure of a lithium-ionsecondary battery (cylindrical type) according to an embodiment of thepresent application;

FIG. 7 is a partially enlarged sectional view of a spirally woundelectrode illustrated in FIG. 6;

FIG. 8 is an exploded perspective view illustrating a structure of alithium-ion secondary battery (laminated-film type) according to anembodiment of the present application;

FIG. 9 is an enlarged cross-sectional view taken along line IX-IX inFIG. 8 that illustrates a spirally wound electrode;

FIG. 10 is a block diagram illustrating the configuration of anapplication example (battery pack) of a lithium-ion secondary battery;

FIG. 11 is a block diagram illustrating the configuration of anapplication example (electric vehicle) of a lithium-ion secondarybattery;

FIG. 12 is a block diagram illustrating the configuration of anapplication example (power storage system) of a lithium-ion secondarybattery; and

FIG. 13 is a block diagram illustrating the configuration of anapplication example (electric tool) of a lithium-ion secondary battery.

DETAILED DESCRIPTION

Embodiments of the present application will be described in detail belowwith reference to the attached drawings. Descriptions are made in theorder listed below:

1. negative electrode for lithium-ion secondary battery,

2. lithium-ion secondary battery,

2-1. prismatic type,

2-2. cylindrical type,

2-3. laminated-film type,

3. application of lithium-ion secondary battery,

3-1. battery pack,

3-2. electric vehicle,

3-3. power storage system, and

3-4. electric tool.

1. Negative Electrode for Lithium-Ion Secondary Battery

FIG. 1 illustrates a sectional structure of a negative electrode for alithium-ion secondary battery (hereinafter, referred to simply as a“negative electrode”) according to an embodiment of the presentapplication. FIG. 2 is a SEM photograph illustrating the cross-sectionalstructure of an active material contained in the negative electrode(negative-electrode active material).

Overall Structure of Negative Electrode

The negative electrode includes, for example, as illustrated in FIG. 1,negative-electrode active material layers 2 on a negative-electrodecollector 1. For this negative electrode, the negative-electrode activematerial layers 2 may be arranged on both surfaces of thenegative-electrode collector 1. Alternatively, one negative-electrodeactive material layer may be arranged on only one surface of thecollector. Furthermore, the negative-electrode collector 1 may not bearranged.

Negative-Electrode Collector

The negative-electrode collector 1 is composed of, for example, aconductive material having excellent electrochemical stability,electrical conductivity, and mechanical strength. Examples of theconductive material include Cu, Ni, and stainless steel. In particular,a material which does not form an intermetallic compound with Li andwhich can be alloyed with a material constituting the negative-electrodeactive material layers 2 is preferred.

The negative-electrode collector 1 preferably contains, as constituentelements, C and S. The reason for this is that the physical strength ofthe negative-electrode collector 1 is improved, so that thenegative-electrode collector 1 is less likely to be deformed even if thenegative-electrode active material layers 2 expand and contract duringcharge and discharge. An example of the negative-electrode collector 1is metal foil doped with C and S. The C content and the S content arenot particularly limited and are each preferably 100 ppm or less becausea higher effect is obtained.

The negative-electrode collector 1 may have a roughened surface or maynot have a roughened surface. An example of the negative-electrodecollector 1 having an unroughened surface is rolled metal foil. Anexample of the negative-electrode collector 1 having a roughened surfaceis metal foil that has been subjected to electrolytic treatment orsandblasting. The electrolytic treatment is a method in which fineparticles are formed on a surface of metal foil or the like in anelectrolytic bath by an electrolytic process to produce irregularities.Metal foil produced by the electrolytic process is commonly referred toas electrolytic foil (e.g., electrolytic Cu foil).

The negative-electrode collector 1 preferably has a roughened surfacebecause the adhesion of the negative-electrode active material layers 2to the negative-electrode collector 1 is improved by an anchor effect.The surface roughness (e.g., ten-point height of irregularities Rz) ofthe negative-electrode collector 1 is not particularly limited and ispreferably maximized in order to improve the adhesion of thenegative-electrode active material layers 2 by the anchor effect.However, an excessively high surface roughness may result in a reductionin the adhesion of the negative-electrode active material layers 2.

Negative-Electrode Active Material Layer

Each of the negative-electrode active material layers 2 contains aplurality of particles of a negative-electrode active material 200capable of occluding and releasing lithium ions as illustrated in FIG.2. If necessary, each of the negative-electrode active material layers 2may further contain an additional material, for example, anegative-electrode binder or a negative-electrode conductive agent.

The negative-electrode active material 200 includes a core portion 201capable of occluding and releasing lithium ions and a covering portion202 arranged on the core portion 201. The state in which the coreportion 201 is covered with the covering portion 202 can be confirmed bySEM observation as illustrated in FIG. 2.

Core Portion

The composition of the core portion 201 is not particularly limited aslong as the core portion 201 is capable of occluding and releasinglithium ions. In particular, the core portion 201 preferably contains,as a constituent element, at least one of Si and Sn because a highenergy density is provided. The core portion 201 may contain Si inelemental form, a Si compound, a Si alloy, or two or more thereof.Similarly, Sn in elemental form, a Sn compound, or a Sn alloy may becontained. The term “elemental form” refers to an elemental form in ageneral sense (the element may contain minute quantities of impurities(elements other than oxygen)) and does not necessarily indicates apurity of 100%.

For example, the Si alloy contains Si and one or two or more elementsselected from Sn, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Cr,and so forth. For example, the Si compound contains Si and one or two ormore elements selected from C, O, and so forth. The Si compound mayfurther contain one or two or more elements described in the Si alloy.Examples of the Si alloy and the Si compound include SiB₄, SiB₆, Mg₂Si,Ni₂Si, TiSi₂, MoSi₂, CoSi₂, NiSi₂, CaSi₂, CrSi₂, Cu₅Si, FeSi₂, MnSi₂,NbSi₂, TaSi₂, VSi₂, WSi₂, ZnSi₂, SiC, Si₃N₄, Si₂N₂O, SiO_(v) (0<v≦2),and LiSiO.

For example, the Sn alloy contains Sn and one or two or more elementsselected from Si, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Cr,and so forth. For example, the Sn compound contains Sn and one or two ormore elements selected from C, O, and so forth. The Sn compound mayfurther contain one or two or more elements described in the Sn alloy.Examples of the Sn alloy and the Sn compound include SnO_(w) (0<w≦2),SnSiO₃, LiSnO, Mg₂Sn, SnCo, SnCoTi, and SnFeCo.

Preferably, the core portion 201 contains, for example, Si and O servingas constituent elements, and the atomic ratio x of O to Si, i.e., O/Si,is 0≦x<0.5. The reason for this is that the core portion 201 easilyoccludes and releases lithium ions during charge and discharge and thata high battery capacity is obtained owing to a reduction in irreversiblecapacity, compared with the case where the atomic ratio x is outside therange (0.5≦x).

As is apparent from the foregoing composition (the atomic ratio x), thecore portion 201 may be composed of elemental Si (x=0) or SiO_(x)(0<x<0.5). Note that x is preferably minimized More preferably, x=0(elemental Si). The reason for this is that a higher energy density isobtained and that the discharge capacity is less likely to decrease fromthe early stage of the charge-discharge cycle because the degradation ofthe core portion 201 is inhibited.

The core portion 201 may have a crystal structure (with high or lowcrystallinity) or an amorphous structure. The core portion 201preferably has a crystal structure with high or low crystallinity andmore preferably with high crystallinity. This is because the coreportion 201 easily occludes and releases lithium ions during charge anddischarge to achieve high battery capacity and so forth and because thecore portion 201 is less likely to expand and contract during charge anddischarge. In particular, in the core portion 201, the half-width (2θ)of a diffraction peak attributed to the silicon (111) crystal faceobserved by X-ray diffraction is preferably 20° or less, and the size ofa crystallite attribute to the (111) crystal face is preferably 10 nm ormore. This is because a higher effect is provided.

The median diameter of the core portion 201 is not particularly limited.In particular, the core portion 201 preferably has a median diameter of0.3 μm to 20 μm because the core portion 201 easily occludes andreleases lithium ions during charge and discharge and because the coreportion 201 is not easily broken. More particularly, a median diameterof less than 0.3 μm can facilitate expansion and contraction duringcharge and discharge because of an excessively large total surface areaof the core portion 201. A median diameter exceeding 20 μm is liable tolead to a break of the core portion 201 during charge and discharge.

The core portion 201 may contain, as a constituent element, one or twoor more additional elements (excluding Si and Sn), together with Si andSn.

Specifically, the core portion 201 preferably contains, as a constituentelement, at least one element M2 selected from Fe and Al. Note that theratio of M2 to Si and O, i.e., M2/(Si+O), is preferably in the range of0.01 atomic percent to 50 atomic percent because the electricalresistance of the core portion 201 is reduced and because thediffusibility of is improved.

In the core portion 201, M2 may be present (in the free state)independently of Si and O or may be combined with at least one of Si andO to form an alloy or a compound. The composition (e.g., the bondingstate of M2) of the core portion 201 including M2 can be identified by,for example, energy dispersive x-ray analysis (EDX). The bonding stateand the identification method of M3 and M4 described below are the sameas described above.

In particular, the core portion 201 preferably contains Al because thecore portion 201 has low crystallinity, so that the core portion 201 isless likely to expand and contract during charge and discharge and thediffusibility of lithium ions is further improved. In the core portion201 containing Al, the half-width (2θ) of a diffraction peak attributedto the Si(111) crystal face observed by X-ray diffraction is preferably0.6° or more. The size of a crystallite attribute to the (111) crystalface is preferably 90 nm or less. In the case where the half-width isinvestigated, preferably, the covering portion 202 is removed bydissolution with HF or the like, and then the core portion 201 isanalyzed.

More particularly, in the case where the core portion 201 does notcontain Al and where the core portion 201 has high crystallinity, thecore portion 201 easily expands and contracts during charge anddischarge. In contrast, in the case where the core portion 201 containsAl, the core portion 201 is less likely to expand and contract duringcharge and discharge regardless of whether the core portion 201 has highor low crystallinity. In this case, when the core portion 201 has lowcrystallinity, the expansion and contraction of the core portion 201 areinhibited, and the diffusibility of lithium ions is improved.

The core portion 201 preferably contains, as a constituent element, atleast one element M3 selected from Cr and Ni. Note that the ratio of M3to Si and O, i.e., M3/(Si+O), is preferably in the range of 1 atomicpercent to 50 atomic percent. Also in this case, the electricalresistance of the core portion 201 is reduced, and the diffusibility oflithium ions is improved.

The core portion 201 preferably contains, as a constituent element, atleast one element M4 selected from B, Mg, Ca, Ti, V, Mn, Co, Cu, Ge, Y,Zr, Mo, Ag, In, Sn, Sb, Ta, W, Pb, La, Ce, Pr, and Nd. Note that theratio of M4 to Si and O, i.e., M4/(Si+O), is preferably in the range of0.01 atomic percent to 30 atomic percent. Also in this case, theelectrical resistance of the core portion 201 is reduced, and thediffusibility of lithium ions is improved.

Covering Portion

The covering portion 202 is arranged on at least part of a surface ofthe core portion 201. Thus, the covering portion 202 may cover part ofthe surface of the core portion 201 or may cover the entire surface ofthe core portion 201. In the case of the former, the covering portion202 may be arranged on a plurality of portions of a surface of the coreportion 201 and may cover the portions.

The covering portion 202 contains, as constituent elements, Si, O, andelement M1 that can form an alloy with Si. Element M1 is at least oneselected from Li, C, Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo,Ag, Sn, Ba, W, Ta, Na, and K. The atomic ratio y of O to Si, i.e., O/Si,is 0.5≦y≦1.8, preferably 0.7≦y≦1.3, and more preferably y=1.2. This isbecause the protective function of the covering portion 202 describedbelow is effectively provided.

The reason the atomic ratio y is within the range described above isthat the degradation of the negative-electrode active material 200 issuppressed when charge and discharge are repeated compared with the casewhere the atomic ratio y is outside the range (i.e., y<0.5, or y>1.8).In this case, the core portion 201 chemically and physically protectsthe core portion 201 while the entry and exit of lithium ions from thecore portion 201 are ensured.

More particularly, in the case where the covering portion 202 intervenesbetween the core portion 201 and an electrolytic solution, the highlyreactive core portion 201 is less likely to come into contact with theelectrolytic solution, thus inhibiting the decomposition reaction of theelectrolytic solution. In this case, when the covering portion 202 iscomposed of a material similar to a material, which contains common Sias a constituent element, constituting the core portion 201, theadhesion of the covering portion 202 to the core portion 201 isincreased.

The covering portion 202 is flexible (easily deformable). Thus, when thecore portion 201 expands and contracts during charge and discharge, thecovering portion 202 follows the deformation to expand and contract(extend and shrink) easily. Hence, if the core portion 201 expands andcontracts, the covering portion 202 is less likely to be damaged(broken). As a result, the state of the core portion 201 covered withthe covering portion 202 is maintained even if charge and discharge arerepeated. Thus, even if the core portion 201 is broken during charge anddischarge, a newly formed surface is less likely to be exposed.Furthermore, the newly formed surface is less likely to come intocontact with an electrolytic solution, thus inhibiting the decompositionreaction of the electrolytic solution.

The reason the covering portion 202 contains M1 together with Si and Ois that when the atomic ratio y is within the range described above, acompound (Si-M1-O) of Si, O, and M1 is easily formed in the coveringportion 202. This results in a reduction in irreversible capacity and areduction in the electrical resistance of the negative-electrode activematerial 200. In the covering portion 202, at least one of the atoms ofM1 may form Si-M1-O. Also in this case, the foregoing advantages areprovided. The remaining M1 may be present in a free elemental form, mayform an alloy with Si, or may be combined with O to form a compound.

More particularly, with respect to the bonding states (valence) of Siatoms bonded to O atoms in the covering portion 202, five valence statesare known: zero valence (Si⁰⁺), monovalence (Si¹⁺), divalence (Si²⁺),trivalence (Si³⁺), and tetravalence (Si⁴⁺). The presence or absence ofSi atoms in these bonding states and their proportions (atomic ratios)can be determined by analysis of the covering portion 202 by, forexample, X-ray photoelectron spectroscopy (XPS). Note that in the casewhere the outermost layer of the covering portion 202 is unintentionallyoxidized (SiO₂ is formed), the analysis is preferably performed afterSiO₂ is removed by dissolution with HF or the like.

In the case where Si-M1-O is formed in the covering portion 202, in thezero-valent to tetra-valent bonding states, the proportion of thetetravalent silicon which is liable to lead to irreversible capacityduring charge and discharge and which has high resistance is relativelyreduced. Thus, even if the covering portion 202 is arranged on thesurface of the core portion 201, the presence of the covering portion202 is less likely to lead to irreversible capacity, and the electricalresistance of the covering portion 202 is reduced.

The ratio of M1 to Si and O, i.e., M1/(Si+O), is not particularlylimited and is preferably 50 atomic percent or less and more preferably20 atomic percent or less. This is because the series of advantages ofthe covering portion 202 described above is obtained while suppressing areduction in battery capacity due to the presence of M1.

As with the core portion 201, the covering portion 202 is preferablycapable of occluding and releasing lithium ions. This is because thecovering portion 202 is less likely to inhibit the occlusion and releaseof lithium ions, so that the core portion 201 easily occludes andreleases lithium ions.

Furthermore, the covering portion 202 is preferably noncrystalline(amorphous) or preferably has low crystallinity. The reason for this isas follows: lithium ions are easily diffused compared with the casewhere the covering portion 202 is crystalline (with high crystallinity),so that even when the surface of the core portion 201 is covered withthe covering portion 202, the core portion 201 easily and smoothlyoccludes and releases lithium ions.

In particular, the covering portion 202 is preferably noncrystalline.This is because the covering portion 202 has improved flexibility andthus easily follows the expansion and contraction of the core portion201 during charge and discharge. Furthermore, the covering portion 202is less likely to trap lithium ions, so that the entry and exit oflithium ions from the core portion 201 are less likely to be inhibited.

The term “low crystallinity” indicates that a material contained in thecovering portion 202 includes a noncrystalline region and a crystallineregion, the material being different from a “noncrystalline” materialincluding a noncrystalline region alone. To identify whether thecovering portion 202 has low crystallinity or not, for example, thecovering portion 202 may be observed with a high-angle annulardark-field scanning transmission electron microscopy (HAADF STEM). If aTEM photograph reveals that a noncrystalline region and a crystallineregion are both present, the covering portion 202 has low crystallinity.In the case where a noncrystalline region and a crystalline region areboth present, the crystalline region is observed as a region (crystalgrain) having a granular contour. A striped pattern (crystal latticepattern) attributed to the crystallinity is observed inside the crystalgrain. It is thus possible to distinguish the crystal grain from thenoncrystalline region.

The covering portion 202 may have a single-layer structure or amultilayer structure. In particular, the covering portion 202 preferablyhas a multilayer structure. This is because the covering portion 202 isless likely to be broken even if the core portion 201 expands andcontracts during charge and discharge. More particularly, for thecovering portion 202 having a single-layer structure, internal stress inthe covering portion 202 is not easily relaxed, depending on itsthickness. Thus, the covering portion 202 can be broken (e.g., fractureor separation) by the effect of the expanded and contracted core portion201 during charge and discharge. In contrast, for the covering portion202 having a multilayer structure, a minute gap between layers functionsas a gap that relaxes stress. Thus, the internal stress is relaxed, sothat the covering portion 202 is not easily broken.

For the covering portion 202 having the single-layer structure, Si, O,and M1 are contained in the single layer. For the covering portion 202having the multilayer structure, layers containing Si, O, and M1 may bestacked. Alternatively, a layer containing Si and O and a layercontaining Si, O, and M1 may be stacked. Furthermore, these layers maybe mixed. Also in this case, the same effect is provided. Of course, anystacking order of the layers may be used in the multilayer structure.

The average thickness of the covering portion 202 is not particularlylimited. In particular, the average thickness is preferably minimized.The covering portion 202 preferably has an average thickness of 1 nm to10,000 nm and more preferably 100 nm to 10,000 nm. This is because thecore portion 201 easily occludes and releases lithium ions and becausethe protective function of the covering portion 202 is effectivelyprovided. More particularly, an average thickness of less than 1 nm cancause the covering portion 202 to be less likely to protect the coreportion 201. An average thickness exceeding 10,000 nm can increase theelectrical resistance and can cause the core portion 201 to be lesslikely to occlude and release lithium ions during charge and discharge.The reason for this is that in the case where the covering portion 202is composed of SiO_(y), SiO_(y) easily occludes lithium ions but doesnot easily release lithium ions that have been occluded.

The average thickness of the covering portion 202 is calculated by thefollowing procedure. As illustrated in FIG. 2, one particle of thenegative-electrode active material 200 is observed with a scanningelectron microscope (SEM). To measure the thickness T of the coveringportion 202, the observation is preferably performed at a magnificationsuch that the boundary between the core portion 201 and the coveringportion 202 can be visually identified (determined). Subsequently,thicknesses of the covering portion 202 are measured at 10 randompositions. Then the average value of the thicknesses (average thicknessT of one particle) is calculated. In this case, the measurementpositions are preferably set in such a manner that the measurementpositions are not localized at a specific site but are widelydistributed to the extent possible. Next, the foregoing calculation ofthe average value is repeated until the total number of particlesobserved with the SEM reaches 100. Finally, the average value (averagevalue of the average thicknesses T) of the calculated averagethicknesses T (corresponding to the respective particles) of 100particles of the negative-electrode active material 200 is calculatedand defined as the average thickness of the covering portion 202.

The average coverage of the covering portion 202 on the core portion 201is not particularly limited and is preferably maximized. Morepreferably, the average coverage of the covering portion 202 ispreferably in the range of 30% to 100% because the protective functionof the covering portion 202 is further improved.

The average coverage of the covering portion 202 is calculated by thefollowing procedure. As with the case where the average thickness iscalculated, one particle of the negative-electrode active material 200is observed with a scanning electron microscope (SEM). The observationis preferably performed at a magnification such that in the core portion201, a portion that is covered with the covering portion 202 and aportion that is not covered with the covering portion 202 can bevisually distinguished. With respect to the outer edge (contour) of thecore portion 201, the length of a portion that is covered with thecovering portion 202 and the length of a portion that is not coveredwith the covering portion 202 are measured. Then the followingcalculation is performed: coverage (coverage for one particle:%)=(length of portion covered with covering portion 202/length of outeredge of core portion 201)×100. Next, the foregoing calculation of thecoverage is repeated until the total number of particles observed withthe SEM reaches 100. Finally, the average value of the calculatedcoverage values (corresponding to the respective particles) of 100particles of the negative-electrode active material 200 is calculatedand defined as the average coverage of the covering portion 202.

The covering portion 202 is preferably adjacent to the core portion 201and may be present on the surface of the core portion 201 with a naturaloxide film (SiO₂) provided therebetween. The natural oxide film isformed by, for example, oxidation of a surface portion of the coreportion 201 in air. In the case where the core portion 201 is present inthe middle of each particle of the negative-electrode active material200 and where the covering portion 202 is present outside the particle,the presence of the natural oxide film has little effect on functions ofthe core portion 201 and the covering portion 202.

To check the fact that the negative-electrode active material 200includes the core portion 201 and the covering portion 202, thenegative-electrode active material 200 may be analyzed by, for example,X-ray photoelectron spectroscopy (XPS) or energy-dispersive X-rayanalysis (EDX) in addition to the SEM observation described above.

In this case, for example, the compositions of the core portion 201 andthe covering portion 202 can be identified by measuring the degrees ofoxidation (atomic ratios x and y) at the central portion and the surfaceportion of each particle of the negative-electrode active material 200.To investigate the composition of the core portion 201 covered with thecovering portion 202, the covering portion 202 may be removed bydissolution with HF or the like.

An exemplary procedure for measuring the degree of oxidation will bedescribed in detail below. First, the negative-electrode active material200 (the core portion 201 covered with the covering portion 202) isquantified by a combustion method to calculate the total amount of Siand the total amount of O. Next, the covering portion 202 is removed byrinsing with HF, and then the core portion 201 is quantified tocalculate the amount of Si and the amount of O. Finally, the amount ofSi and the amount of O in the covering portion 202 are determined bysubtracting the amount of Si and the amount of O in the core portion 201from the total amount of Si and the total amount of O. As a result, theamounts of Si and the amounts of O in the core portion 201 and thecovering portion 202 are determined, thus determining the degrees ofoxidation therein. In place of the removal of the covering portion 202by rinsing, the degrees of oxidation may be measured by the use of thecore portion 201 covered with the covering portion 202 and a portion ofthe core portion 201 that is not covered therewith.

Conductive Portion

In particular, the negative-electrode active material 200 may include aconductive portion on a surface of the covering portion 202. Theconductive portion is arranged on at least part of a surface of thecovering portion 202 and has a lower electrical resistance than those ofthe core portion 201 and the covering portion 202. In this case, thecore portion 201 does not easily come into contact with an electrolyticsolution, thus inhibiting the decomposition reaction of the electrolyticsolution. Furthermore, the electrical resistance of thenegative-electrode active material 200 is further reduced. Theconductive portion contains, for example, one or two or more of carbonmaterials, metal materials, and inorganic compounds. An example ofcarbon materials is graphite. Examples of metal materials include Fe,Cu, and Al. An example of inorganic compounds is SiO₂. Among thesematerials, carbon materials or metal materials are preferred. Carbonmaterials are more preferred. This is because the electrical resistanceof the negative-electrode active material 200 is further reduced. Notethat the conductive portion may have any average coverage and anyaverage thickness. The average coverage and the average thickness arecalculated in the same ways as those used for the covering portion 202.

The negative-electrode binder contains, for example, one or two or moreof synthetic rubber and polymeric materials. Examples of syntheticrubber include styrene-butadiene-based rubber, fluorocarbon rubber, andethylene-propylene-diene. Examples of polymeric materials includepolyvinylidene fluoride, polyimide, polyamide, polyamide-imide,polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polymaleicacid, and copolymers thereof. Further examples of polymeric materialsinclude carboxymethyl cellulose, styrene-butadiene rubber, and polyvinylalcohol.

The negative-electrode conductive agent contains, for example, one ortwo or more of carbon materials, such as graphite, carbon black,acetylene black, and Ketjenblack. The negative-electrode conductiveagent may also be a conductive material, for example, a metal materialor a conductive polymer.

Each of the negative-electrode active material layers 2 may containanother negative-electrode active material in addition to thenegative-electrode active material 200 including the core portion 201and the covering portion 202 described above, if necessary.

An example of another negative-electrode active material is a carbonmaterial. This is because the electrical resistance of thenegative-electrode active material layers 2 is reduced and because thenegative-electrode active material layers 2 are less likely to expandand contract during charge and discharge. Examples of the carbonmaterial include graphitizable carbon, non-graphitizable carbon in whichthe interplanar spacing of the (002) planes is 0.37 nm or more, andgraphite in which the interplanar spacing of the (002) planes is 0.34 nmor less. Specific examples thereof include pyrolytic carbon, coke,glassy carbon fibers, a burned organic polymeric compound, activatedcarbon, and carbon black. Examples of coke include pitch coke, needlecoke, and petroleum coke. The burned organic polymeric compound refersto a material formed by burning a phenolic resin or a furan resin at anappropriate temperature into carbon. The carbon material may have anyshape selected from fibrous shapes, spherical shapes, granular shapes,and flaky shapes. The carbon material content of the negative-electrodeactive material layers 2 is not particularly limited and is preferably60% by weight or less and more preferably 10% by weight to 60% byweight.

Furthermore, another negative-electrode active material may be a metaloxide or a polymeric compound. Examples of the metal oxide include ironoxide, ruthenium oxide, and molybdenum oxide. Examples of the polymericcompound include polyacetylene, polyaniline, and polypyrrole.

The negative-electrode active material layers 2 are formed by, forexample, an application method, a firing method (a sintering method), ora combination of two or more thereof. The application method refers to amethod in which, for example, a negative-electrode active material ismixed with a negative-electrode binder, the resulting mixture isdispersed in an organic solvent, and application is performed. Thefiring method refers to a method in which, for example, afterapplication is performed in the same way as the application method, heattreatment is performed at a temperature higher than the melting point ofthe negative-electrode binder or the like. As the firing method, amethod of the related art may be employed. Examples thereof include anatmosphere firing method, a reactive firing method, and a hot-pressfiring method.

Production Method of Negative Electrode

A negative electrode is produced by, for example, a procedure describedbelow. The materials constituting the negative-electrode collector 1 andthe negative-electrode active material layers 2 have been described indetail. Thus, the descriptions are appropriately omitted.

First, the granular (powdery) core portion 201 having the foregoingcomposition is formed by, for example, a gas atomization method, a wateratomization method, or a melt pulverization method.

Next, the covering portion 202 having the foregoing composition isformed on the surface of the core portion 201 by a vapor depositionmethod, for example, an evaporation method or a sputtering method. Inthe case where a material constituting the covering portion 202 isdeposited by the vapor deposition method as described above, thecovering portion 202 tends to be noncrystalline. In this case, thematerial constituting the covering portion 202 may be deposited whilebeing heated by, for example, induction heating, resistance heating, orelectron-beam heating. Alternatively, after the formation of thecovering portion 202, the covering portion 202 may be heated so as tohave low crystallinity. The degree of the low crystallinity iscontrolled, depending on, for example, heating conditions, such astemperature and time. The heat treatment results in the removal of waterin the covering portion 202 and results in improvement in the adhesionof the covering portion 202 to the core portion 201.

In particular, in the case where the vapor deposition method isemployed, Si-M1-O is easily formed in the covering portion 202 byheating not only the material constituting the covering portion 202 butalso a substrate used for the deposition. For example, the substratetemperature is preferably 200° C. or higher and lower than 900° C. Whenthe covering portion 202 is formed, the proportions of the bondingstates of Si atoms bonded to O atoms can be controlled by adjusting theflow rates of oxygen (O₂), hydrogen (H₂), and so forth introduced into achamber and by adjusting the temperature of the core portion 201. As aresult, the core portion 201 is covered with the core portion 201,resulting in the negative-electrode active material 200.

In the case where the negative-electrode active material 200 is formed,a conductive portion may be formed on the surface of the coveringportion 202 by, for example, a vapor deposition method, such as anevaporation method, a sputtering method, or a chemical vapor deposition(CVD) method, or a wet coating method.

In the case where the evaporation method is employed, for example, vaporis allowed to impinge directly on the surface of the negative-electrodeactive material 200. In the case where the sputtering method isemployed, for example, a conductive portion is formed by a powdersputtering method while Ar gas is introduced. In the case where the CVDmethod is employed, for example, a gas formed by subliming a metalchloride and a mixed gas of H₂, N₂, and so forth are mixed in such amanner that the mole fraction of the metal chloride is in the range of0.03 to 0.3, and then the resulting gas is heated to 1000° C. or higherto form a conductive portion on the surface of the covering portion 202.In the case where the wet coating method is employed, for example, analkali solution is added to a slurry containing the negative-electrodeactive material 200 while a metal-containing solution is added to theslurry to form a metal hydroxide. Then reduction treatment with H₂ isperformed at 450° C. to form a conductive portion on the surface of thecovering portion 202. In the case where a carbon material is used as amaterial constituting the conductive portion, the negative-electrodeactive material 200 is placed in a chamber. An organic gas is introducedinto the chamber. Heat treatment is performed at 10,000 Pa and 1000° C.or higher for 5 hours to form a conductive portion on the surface of thecovering portion 202. The organic gas is not particularly limited aslong as it is thermally decomposed to form carbon. Examples of theorganic gas include methane, ethane, ethylene, acetylene, and propane.

Next, the negative-electrode active material 200 and other materials,such as the negative-electrode binder, are mixed to form anegative-electrode mixture. The resulting negative-electrode mixture isdissolved in a solvent, such as an organic solvent, to form a slurrycontaining the negative-electrode mixture. Finally, the slurrycontaining the negative-electrode mixture is applied to the surface ofthe negative-electrode collector 1 and dried to form thenegative-electrode active material layers 2. If necessary, thenegative-electrode active material layers 2 may be subjected tocompression forming and heated (fired).

Function and Effect of Embodiment

In the negative electrode, the negative-electrode active material 200includes the covering portion 202 on the surface of the core portion201. The covering portion 202 contains, as constituent elements, Si, O,and element M1, such as Li. The atomic ratio y of O to Si is 0.5≦y≦1.8.Thus, the core portion 201 easily and smoothly occludes and releaseslithium ions. Furthermore, the core portion 201 is protected by thecovering portion 202 so as not to expose a newly-formed surface duringcharge and discharge while the smooth occlusion and release aremaintained. Moreover, Si-M-O is easily formed in the covering portion202. Thus, the presence of the covering portion 202 is less likely tolead to irreversible capacity, and the electrical resistance of thecovering portion 202 is reduced. Accordingly, the negative electrodecontributes to improvement in the performance of a lithium-ion secondarybattery including the negative electrode. Specifically, the negativeelectrode contributes to improvement in cycle characteristics, initialcharge-discharge characteristics, load characteristics, and so forth.

In particular, in the case where the ratio of M1 to Si and O in thecovering portion 202 is preferably 50 atomic percent or less and morepreferably 20 atomic percent or less, a higher effect can be provided.In the case where the covering portion 202 on the core portion 201 hasan average coverage of 30% to 100% or where the covering portion 202 hasan average thickness of 1 nm to 10,000 nm, a higher effect can beprovided. Moreover, the covering portion 202 has a multilayer structure,a higher effect can be provided.

2. Lithium-Ion Secondary Battery

A lithium-ion secondary battery including the negative electrode for alithium-ion secondary battery (hereinafter, referred to simply as“secondary battery”) will be described below.

2-1. Prismatic Type

FIGS. 3 and 4 are sectional views illustrating a structure of aprismatic lithium-ion secondary battery. FIG. 4 is a sectional viewtaken along line IV-IV in FIG. 3. FIG. 5 is a schematic plan viewillustrating structures of a positive electrode 21 and a negativeelectrode 22 illustrated in FIG. 4.

Entire Structure of Secondary Battery

The prismatic secondary battery mainly includes a battery element 20 ina battery can 11. The battery element 20 is formed of a spirally woundlaminate in which the positive electrode 21 and the negative electrode22 are stacked with a separator 23 provided therebetween and spirallywound, the battery element 20 having a flattened shape in response tothe shape of the battery can 11.

The battery can 11 is, for example, a prismatic package member. Asillustrated in FIG. 4, the prismatic package member has a rectangular orsubstantially rectangular (partially curved) shape in longitudinalsection. The prismatic package member can be used to not only aprismatic battery with a rectangular shape but also a prismatic batterywith an oval shape. In other words, the prismatic package member is avessel-shaped member having a rectangular closed end or an oval closedend and having an opening portion with a rectangular shape or asubstantially rectangular (oval) shape formed by connecting arcs withstraight lines. FIG. 4 illustrates the battery can 11 having arectangular section.

The battery can 11 is composed of a conductive material, for example,iron, aluminum, or an alloy thereof. The battery can 11 may function asan electrode terminal. Among these materials, Fe, which is harder thanAl, is preferred in order to prevent swelling of the battery can 11during charge and discharge with use of the hardness (resistance todeformation) of the battery can 11. In the case where the battery can 11is composed of Fe, the surface of the battery can 11 may be plated withNi or the like.

The battery can 11 has a hollow structure having an open end portion anda closed end portion. The battery can 11 is sealed with an insulatingplate 12 and a battery cover 13 attached to the open end portion. Theinsulating plate 12 is arranged between the battery element 20 and thebattery cover 13. The insulating plate 12 is composed of an insulatingmaterial, such as polypropylene. The battery cover 13 is composed of,for example, a material the same as that of the battery can 11.Similarly to the battery can 11, the battery cover 13 may function as anelectrode terminal.

A terminal plate 14 serving as a positive-electrode terminal is arrangedoutside the battery cover 13. The terminal plate 14 is electricallyinsulated from the battery cover 13 with an insulating case 16 providedtherebetween. The insulating case 16 is composed of an insulatingmaterial, such as polybutylene terephthalate. A through hole is arrangedin the substantially middle of the battery cover 13. Apositive-electrode pin 15 is interposed in the through hole so as to beelectrically connected to the terminal plate 14 and so as to beelectrically insulated from the battery cover 13 with a gasket 17. Thegasket 17 is composed of, for example, an insulating material. Thesurface of the gasket 17 is coated with asphalt.

A cleavage valve 18 and an injection hole 19 are arranged at the outeredge of the battery cover 13. The cleavage valve 18 is electricallyconnected to the battery cover 13. If the internal pressure of thebattery is increased to a predetermined value or higher by an internalshort-circuit or externally applied heat, the cleavage valve 18 isseparated from the battery cover 13 to release the internal pressure.The injection hole 19 is capped with a sealing member 19A formed of, forexample, a stainless-steel ball.

A positive-electrode lead 24 composed of a conductive material, such asAl, is attached to an end portion (for example, an inner end portion) ofthe positive electrode 21. A negative-electrode lead 25 composed of aconductive material, such as Ni, is attached to an end portion (forexample, an outer end portion) of the negative electrode 22. Thepositive-electrode lead 24 is welded to an end of the positive-electrodepin 15 and is electrically connected to the terminal plate 14. Thenegative-electrode lead 25 is welded and electrically connected to thebattery can 11.

Positive Electrode

For example, the positive electrode 21 includes a positive-electrodeactive material layer 21B provided on each surface of apositive-electrode collector 21A. Alternatively, the positive-electrodeactive material layer 21B may be arranged on only one surface of thepositive-electrode collector 21A.

The positive-electrode collector 21A is composed of a conductivematerial, e.g., Al, Ni, or stainless steel.

Each of the positive-electrode active material layers 21B contains oneor two or more positive-electrode materials which serve aspositive-electrode active materials and which are capable of occludingand releasing lithium ions. If necessary, each of the positive-electrodeactive material layers 21B may further contain an additional material,for example, a positive-electrode binder or a positive-electrodeconductive agent. Details of the positive-electrode binder and thepositive-electrode conductive agent are the same as those of thenegative-electrode binder and the negative-electrode conductive agentdescribed above.

As the positive-electrode material, a Li-containing compound ispreferred because a high energy density is provided. Examples of theLi-containing compound include a complex oxide that contains, asconstituent elements, Li and a transition metal element; and a phosphatecompound that contains, as constituent elements, Li and a transitionmetal element. It is preferred that the transition metal element be oneor two or more of Co, Ni, Mn, and Fe. This is because a higher voltageis provided. The complex oxide and the phosphate compound are expressedas, for example, Li_(x)M₁₁O₂ and Li_(y)M₁₂PO₄, respectively, wherein M11and M12 each represent one or more transition metal elements, and thevalues of x and y vary depending on a charge-discharge state of thebattery and are usually 0.05≦x≦1.10 and 0.05≦y≦1.10. In particular, whenthe positive-electrode material contains Ni or Mn, the volume stabilitytends to be improved.

Examples of the complex oxide containing Li and a transition metalelement include Li_(x)CoO₂, Li_(x)NiO₂, and a LiNi-based complex oxiderepresented by formula (1). Examples of the phosphate compoundcontaining Li and a transition metal element include LiFePO₄ andLiFe_(1-u)Mn_(u)PO₄ (u<1). In this case, a high battery capacity andexcellent cycle characteristics are provided. The positive-electrodematerial may be a material other than the foregoing materials. Examplesthereof include materials represented by Li_(x)M_(14y)O₂ (wherein M14represents at least one selected from Ni and M13 described in formula(1); x>1; and y represents any value).LiNi_(1-x)M13_(x)O₂  (1)

(wherein M13 represents at least one selected from Co, Mn, Fe, Al, V,Sn, Mg, Ti, Sr, Ca, Zr, Mo, Tc, Ru, Ta, W, Re, Y, Cu, Zn, Ba, B, Cr, Si,Ga, P, Sb, and Nb; and x is 0.005<x<0.5).

Further examples of the positive-electrode material include oxides,disulfides, chalcogenides, and conductive polymers. Examples of oxidesinclude titanium oxide, vanadium oxide, and manganese dioxide. Examplesof disulfides include titanium disulfide and molybdenum sulfide. Anexample of chalcogenides is niobium selenide. Examples of conductivepolymers include sulfur, polyaniline, and polythiophene.

Negative Electrode

The negative electrode 22 has the same structure as that of theforegoing negative electrode for a lithium-ion secondary battery. Forexample, the negative electrode 22 includes a negative-electrode activematerial layer 22B provided on each surface of a negative-electrodecollector 22A. The structures of the negative-electrode collector 22Aand the negative-electrode active material layers 22B are the same asthose of the negative-electrode collector 1 and the negative-electrodeactive material layers 2. The chargeable capacity of thenegative-electrode material capable of occluding and releasing lithiumions is preferably higher than the discharge capacity of the positiveelectrode 21 in order to prevent metallic Li from being unintentionallydeposited during charge and discharge.

As illustrated in FIG. 5, for example, each of the positive-electrodeactive material layers 21B is arranged on a portion (for example, amiddle region in the longitudinal direction) of a corresponding one ofthe surfaces of the positive-electrode collector 21A. Meanwhile, forexample, each of the negative-electrode active material layers 22B isarranged on the whole of a corresponding one of the surfaces of thenegative-electrode collector 22A. Thus, each of the negative-electrodeactive material layers 22B is arranged in a region (opposed region R1)of the negative-electrode collector 22A which is opposed to acorresponding one of the positive-electrode active material layers 21B,and a region (non-opposed region R2) of the negative-electrode collector22A which is not opposed to the corresponding positive-electrode activematerial layer 21B. In this case, a portion of each negative-electrodeactive material layer 22B arranged in the opposed region R1 isassociated with charge and discharge, whereas a portion arranged in thenon-opposed region R2 has little association with charge and discharge.In FIG. 5, shaded areas indicate the positive-electrode active materiallayer 21B and the negative-electrode active material layer 22B.

As described above, the negative-electrode active material 200 (see FIG.2) contained in the negative-electrode active material layers 22Bincludes the core portion 201 and the covering portion 202. However, thenegative-electrode active material layers 22B might be deformed orbroken by expansion and contraction during charge and discharge. Thus,the formation states of the core portion 201 and the covering portion202 may be changed from states at the time of the formation of thenegative-electrode active material layers 22B. However, in thenon-opposed region R2, the formation states of the negative-electrodeactive material layers 22B are little affected by charge and dischargeand are maintained. Thus, the foregoing parameters, such as the presenceor absence of the core portion 201 and the covering portion 202, thecompositions (atomic ratios x and y) of the core portion 201 and thecovering portion 202, and the proportion of M1, are preferablyinvestigated in the negative-electrode active material layers 22B in thenon-opposed region R2. This is because the presence or absence of thecore portion 201 and the covering portion 202, the compositions of thecore portion 201 and the covering portion 202, and so forth can beinvestigated reproducibly and accurately independent of charge-dischargehistory (e.g., whether charge and discharge are performed or not, andthe number of times of charge and discharge).

The maximum utilization factor in a fully charged state of the negativeelectrode 22 (hereinafter, referred to simply as a “negative-electrodeutilization factor”) is not particularly limited. Any negative-electrodeutilization factor may be set in response to the ratio of the capacityof the positive electrode 21 to the capacity of the negative electrode22.

The foregoing “negative-electrode utilization factor” is expressed asutilization factor Z (%)=(X/Y)×100, where X represents the amount oflithium ions occluded in the negative electrode 22 per unit area in afully charged state, and Y represents the amount of lithium ions thatcan be electrochemically occluded in the negative electrode 22 per unitarea.

For example, the amount X occluded can be determined by the followingprocedure. First, the secondary battery is charged to a fully chargedstate. The secondary battery is then disassembled to cut out a portion(test piece of the negative electrode) of the negative electrode 22opposed to the positive electrode 21. Next, an evaluation batteryincluding metallic lithium serving as a counter electrode is assembledusing the test piece of the negative electrode. Finally, the evaluationbattery is discharged to measure the discharge capacity at the time ofinitial discharge. The discharge capacity is divided by the area of thetest piece of the negative electrode to determine the amount X occluded.In this case, the term “discharge” indicates that a current flows in adirection where lithium ions are released from the test piece of thenegative electrode. For example, the battery is subjected to constantcurrent discharge at a constant current, for example, at a currentdensity of 0.1 mA/cm², until the battery voltage reaches 1.5 V.

Meanwhile, for example, the amount Y occluded is determined bysubjecting the foregoing discharged evaluation battery to constantvoltage and constant current charge until the battery voltage reaches 0V, measuring the charge capacity, and dividing the charge capacity bythe area of the test piece of the negative electrode. In this case, theterm “charge” indicates that a current flows in a direction wherelithium ions are occluded in the test piece of the negative electrode.For example, the battery is subjected to constant voltage charge at acurrent density of 0.1 mA/cm² and a battery voltage of 0 V until thecurrent density reaches 0.02 mA/cm².

In particular, the negative-electrode utilization factor is preferablyin the range of 35% to 80% because excellent cycle characteristics,initial charge-discharge characteristics, and load characteristics areprovided.

Separator

The separator 23 isolates the positive electrode 21 from the negativeelectrode 22 to prevent the occurrence of a short circuit due to thecontact of both electrodes and allows lithium ions to pass therethrough.The separator 23 is formed of, for example, a porous film of a syntheticresin or a ceramic material. The separator 23 may be formed of alaminated film in which two or more porous films are stacked. Examplesof the synthetic resin include polytetrafluoroethylene, polypropylene,and polyethylene.

Electrolytic Solution

The separator 23 is impregnated with an electrolytic solution, which isa liquid electrolyte. The electrolytic solution is formed by dissolutionof an electrolyte salt in a solvent. The electrolytic solution maycontain an additional material, such as an additive, if necessary.

The solvent contains one or two or more of nonaqueous solvents, such asorganic solvents. Examples of nonaqueous solvents include ethylenecarbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate,γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran,2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane,4-methyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, methyl acetate, ethylacetate, methyl propionate, ethyl propionate, methyl butyrate, methylisobutyrate, methyl trimethylacetate, ethyl trimethylacetate,acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile,3-methoxypropionitrile, N,N-dimethylformamide, N-methylpyrrolidinone,N-methyloxazolidinone, N,N′-dimethylimidazolidinone, nitromethane,nitroethane, sulfolane, trimethyl phosphate, and dimethyl sulfoxide. Inthis case, excellent battery capacity, cycle characteristics, andstorage characteristics are provided.

In particular, at least one selected from ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, and ethyl methylcarbonate is preferred because superior characteristics are provided. Inthis case, a combination of a high-viscosity (high dielectric constant)solvent (for example, dielectric constant ∈≧30), e.g., ethylenecarbonate or propylene carbonate, and a low-viscosity solvent (forexample, viscosity≦1 mPa·s), e.g., dimethyl carbonate, ethyl methylcarbonate, or diethyl carbonate, is more preferred because thedissociation property of the electrolyte salt and ion mobility areimproved.

In particular, the nonaqueous solvent preferably contains at least oneof a halogenated chain carbonate and a halogenated cyclic carbonate.This is because a stable coating film is formed on a surface of thenegative electrode 22 during charge and discharge, thereby inhibitingthe decomposition reaction of the electrolytic solution. The halogenatedchain carbonate indicates a chain carbonate containing a halogen servingas a constituent element. In other words, the halogenated chaincarbonate indicates a chain carbonate in which at least one hydrogenatom is substituted with a halogen. The halogenated cyclic carbonateindicates a cyclic carbonate containing a halogen serving as aconstituent element. In other words, the halogenated cyclic carbonateindicates a cyclic carbonate in which at least one H is substituted witha halogen.

The type of halogen is not particularly limited. In particular, F, Cl,or Br is preferred, and F is more preferred. This is because F providesa higher effect than those of other halogens. With respect to the numberof halogen atoms, two halogen atoms are more preferable than one halogenatom. Furthermore, three or more halogen atoms may be used. The reasonfor this is that the ability to form a protective film is increased anda stronger and stabler coating film is formed, thereby furtherinhibiting the decomposition reaction of the electrolytic solution.

Examples of the halogenated chain carbonate include fluoromethyl methylcarbonate, bis(fluoromethyl) carbonate, and difluoromethyl methylcarbonate. Examples of the halogenated cyclic carbonate include4-fluoro-1,3-dioxolan-2-one, and 4,5-difluoro-1,3-dioxolan-2-one. Forthe halogenated cyclic carbonate, geometrical isomers are included. Theproportions of the halogenated chain carbonate and the halogenatedcyclic carbonate in the nonaqueous solvent are each in the range of, forexample, 0.01% by weight to 50% by weight.

The nonaqueous solvent preferably contains an unsaturated carbon bondcyclic carbonate. This is because a stable coating film is formed on asurface of the negative electrode 22 during charge and discharge toinhibit the decomposition reaction of the electrolytic solution. Theunsaturated carbon bond cyclic carbonate indicates a cyclic carbonatehaving one or two or more unsaturated carbon bonds. In other words, theunsaturated carbon bond cyclic carbonate indicates a cyclic carbonate inwhich an unsaturated carbon bond is introduced into any portion.Examples of the unsaturated carbon bond cyclic carbonate includevinylene carbonate and vinyl ethylene carbonate. The proportion of theunsaturated carbon bond cyclic carbonate in the nonaqueous solvent is inthe range of, for example, 0.01% by weight to 10% by weight.

The nonaqueous solvent preferably contains a sultone (cyclic sulfonate)because the chemical stability of the electrolytic solution is improved.Examples of the sultone include propane sultone and propene sultone. Theproportion of the sultone is in the range of, for example, 0.5% byweight to 5% by weight.

The nonaqueous solvent preferably contains an acid anhydride because thechemical stability of the electrolytic solution is improved. Examples ofthe acid anhydride include carboxylic anhydrides, disulfonic anhydrides,and carboxylic-sulfonic anhydrides. Examples of carboxylic anhydridesinclude succinic anhydride, glutaric anhydride, and maleic anhydride.Examples of disulfonic anhydrides include ethanedisulfonic anhydride andpropanedisulfonic anhydride. Examples of carboxylic-sulfonic anhydridesinclude sulfobenzoic anhydride, sulfopropionic anhydride, andsulfobutyric anhydride. The proportion of the acid anhydride in thenonaqueous solvent is in the range of, for example, 0.5% by weight to 5%by weight.

The electrolyte salt contains one or two or more light metal salts, suchas Li salts. Examples of Li salts include LiPF₆, LiBF₄, LiClO₄, LiAsF₆,LiB(C₆H₅)₄, LiCH₃SO₃, LiCF₃SO₃, LiAlCl₄, Li₂SiF₆, LiCl, and LiBr.Another Li salt may be used. This is because excellent battery capacity,cyclic characteristics, storage characteristics, and so forth areprovided.

Among these compounds, one or two or more of LiPF₆, LiBF₄, LiClO₄, andLiAsF₆ are preferred. LiPF₆ or LiBF₄ is preferred. LiPF₆ is morepreferred. This is because the internal resistance is reduced to achievesuperior characteristics.

The electrolyte salt content is preferably in the range of 0.3 mol/kg to3.0 mol/kg with respect to the solvent because a high ionic conductivityis provided.

Operation of Secondary Battery

When the prismatic secondary battery is charged, for example, lithiumions released from the positive electrode 21 are occluded in thenegative electrode 22 through the electrolytic solution. When theprismatic secondary battery is discharged, for example, lithium ionsreleased from the negative electrode 22 are occluded in the positiveelectrode 21 through the electrolytic solution.

Method for Producing Secondary Battery

The secondary battery is produced by, for example, a procedure describedbelow.

First, the positive electrode 21 is formed. The positive-electrodeactive material and, if necessary, the positive-electrode binder, thepositive-electrode conductive agent, and so forth are mixed together toform a positive-electrode mixture. The positive-electrode mixture isdispersed in an organic solvent or the like to form a paste-like slurryof the positive-electrode mixture. Next, the paste-like slurry of thepositive-electrode mixture is applied to the positive-electrodecollector 21A with a coating apparatus, for example, a doctor blade or abar coater, and dried to form the positive-electrode active materiallayers 21B. Finally, the positive-electrode active material layers 21Bare subjected to compression forming with a roll press or the like whilebeing heated, if necessary. In this case, the compression forming may berepeated plural times.

Next, the negative-electrode active material layers 22B are formed onthe negative-electrode collector 22A by a procedure the same as theforegoing procedure for forming the negative electrode 22 for alithium-ion secondary battery.

Next, the battery element 20 is formed. First, the positive-electrodelead 24 is attached to the positive-electrode collector 21A by a weldingmethod or the like. The negative-electrode lead 25 is attached to thenegative-electrode collector 22A by a welding method or the like. Thenthe positive electrode 21 and the negative electrode 22 are stacked withthe separator 23 provided therebetween. The resulting stack is spirallywound in a longitudinal direction. Finally, the resulting spirally woundbody is formed so as to have a flattened shape.

Finally, the secondary battery is assembled. First, the battery element20 is placed in the battery can 11. The insulating plate 12 is placed onthe battery element 20. Next, the positive-electrode lead 24 is attachedto the positive-electrode pin 15 by a welding method or the like. Thenegative-electrode lead 25 is attached to the battery can 11 by awelding method or the like. In this case, the battery cover 13 is fixedto an open end portion of the battery can 11 by a laser welding methodor the like. Finally, the electrolytic solution is injected into thebattery can 11 from the injection hole 19 to impregnate the separator 23with the electrolytic solution, and then the injection hole 19 is sealedwith the sealing member 19A.

Function and Effect of Secondary Battery

For the prismatic secondary battery, the negative electrode 22 has thesame structure as that of the foregoing negative electrode for alithium-ion secondary battery, thereby providing the same effects. It isthus possible to provide excellent battery characteristics, such ascycle characteristics, initial charge-discharge characteristics, andload characteristics. Effects other than these effects are the same asthose of the negative electrode for a lithium-ion secondary battery.

2-2. Cylindrical Type

FIGS. 6 and 7 are sectional views illustrating a structure of acylindrical lithium-ion secondary battery. FIG. 7 is a partiallyenlarged view of a spirally wound electrode 40 illustrated in FIG. 6.The cylindrical secondary battery will be described below with referenceto components of the foregoing prismatic secondary battery, asnecessary.

Structure of Secondary Battery

The cylindrical secondary battery mainly includes the spirally woundelectrode 40 and a pair of insulating plates 32 and 33 in asubstantially hollow cylindrical-shaped battery can 31. The spirallywound electrode 40 is formed of a spirally wound laminate in which apositive electrode 41 and a negative electrode 42 are stacked with aseparator 43 provided therebetween and spirally wound.

The battery can 31 has a hollow structure in which an end portion of thebattery can 31 is closed and the other end portion thereof is opened.The battery can 31 is composed of, for example, a material the same asthat of the battery can 11. The pair of insulating plates 32 and 33 arearranged in such a manner that the spirally wound electrode 40 issandwiched therebetween at the top and the bottom of the spirally woundelectrode body 40 and that the pair of insulating plates 32 and 33extend in a direction perpendicular to a peripheral winding surface.

In the open end portion of the battery can 31, a battery cover 34, asafety valve mechanism 35, and a positive temperature coefficient device(PTC device) 36 are caulked with a gasket 37. The battery can 31 issealed. The battery cover 34 is composed of, for example, a material thesame as that of the battery can 31. The safety valve mechanism 35 andthe positive temperature coefficient device 36 are arranged inside thebattery cover 34. The safety valve mechanism 35 is electricallyconnected to the battery cover 34 through the positive temperaturecoefficient device 36. In the safety valve mechanism 35, if the internalpressure of the secondary battery is increased to a predetermined valueor higher by an internal short-circuit or externally applied heat, adisk plate 35A is reversed to disconnect the electrical connectionbetween the battery cover 34 and the spirally wound electrode 40. Thepositive temperature coefficient device 36 exhibits an increase inresistance with increasing temperature and thus prevents abnormal heatgeneration due to a large current. The gasket 37 is composed of, forexample, an insulating material. The surface of the gasket 37 may becoated with asphalt.

A center pin 44 may be interposed in the center of the spirally woundelectrode 40. A positive-electrode lead 45 composed of a conductivematerial, such as Al, is connected to the positive electrode 41. Anegative-electrode lead 46 composed of a conductive material, such asNi, is connected to the negative electrode 42. The positive-electrodelead 45 is attached to the safety valve mechanism 35 by welding or thelike to establish electrical connection with the battery cover 34. Thenegative-electrode lead 46 is attached to the battery can 31 by weldingor the like to establish electrical connection with the battery can 31.

The positive electrode 41 includes, for example, a positive-electrodeactive material layer 41B provided on each surface of apositive-electrode collector 41A. The negative electrode 42 has the samestructure as the foregoing negative electrode for a lithium-ionsecondary battery. For example, the negative electrode 42 includes anegative-electrode active material layer 42B provided on each surface ofa negative-electrode collector 42A. The structures of thepositive-electrode collector 41A, the positive-electrode active materiallayers 41B, the negative-electrode collector 42A, the negative-electrodeactive material layers 42B, and the separator 43 are the same as thoseof the positive-electrode collector 21A, the positive-electrode activematerial layers 21B, the negative-electrode collector 22A, thenegative-electrode active material layers 22B, and the separator 23,respectively. The composition of an electrolytic solution with which theseparator 43 is impregnated is the same as that of the electrolyticsolution used in the prismatic secondary battery.

Operation of Secondary Battery

When the cylindrical secondary battery is charged, for example, lithiumions released from the positive electrode 41 are occluded in thenegative electrode 42 through the electrolytic solution. When thecylindrical secondary battery is discharged, for example, lithium ionsreleased from the negative electrode 42 are occluded in the positiveelectrode 41 through the electrolytic solution.

Method for Producing Secondary Battery

The cylindrical secondary battery is produced by, for example, aprocedure described below. First, for example, the positive-electrodeactive material layer 41B is formed on each of the surfaces of thepositive-electrode collector 41A to form the positive electrode 41 inthe same way as the procedure for forming the positive electrode 21. Thenegative-electrode active material layer 42B is formed on each of thesurfaces of the negative-electrode collector 42A to form the negativeelectrode 42 in the same way as the procedure for forming the negativeelectrode 22. Next, the positive-electrode lead 45 is attached to thepositive electrode 41 by a welding method or the like. Thenegative-electrode lead 46 is attached to the negative electrode 42 by awelding method or the like. Then the positive electrode 41 and thenegative electrode 42 are stacked with the separator 43 providedtherebetween. The resulting stack is spirally wound to form a spirallywound electrode 40. The center pin 44 is inserted into the center of thespirally wound electrode 40. The spirally wound electrode 40 is placedin the battery can 31 while being sandwiched between the pair ofinsulating plates 32 and 33. In this case, the positive-electrode lead45 is attached to the safety valve mechanism 35 by a welding method orthe like. An end portion of the negative-electrode lead 46 is attachedto the battery can 31 by a welding method or the like. Subsequently, theelectrolytic solution is injected into the battery can 31, therebyimpregnating the separator 43 with the electrolytic solution. Finally,the battery cover 34, the safety valve mechanism 35, and the positivetemperature coefficient device 36 are attached to the open end portionof the battery can 31 and then are caulked with the gasket 37.

Function and Effect of Secondary Battery

For the cylindrical secondary battery, the negative electrode 42 has thesame structure as that of the foregoing negative electrode for alithium-ion secondary battery, thereby providing the same effects asthose of the prismatic secondary battery.

2-3. Laminated-Film Type

FIG. 8 is an exploded perspective view illustrating a structure of alaminated-film-type lithium-ion secondary battery. FIG. 9 is an enlargedcross-sectional view taken along line IX-IX in FIG. 8 that illustrates aspirally wound electrode 50.

Structure of Secondary Battery

The laminated-film-type secondary battery mainly includes the spirallywound electrode 50 in film-shaped package members 59. The spirally woundelectrode 50 is formed of a spirally wound laminate in which a positiveelectrode 53 and a negative electrode 54 are stacked with separators 55and electrolyte layers 56 provided therebetween and spirally wound. Apositive-electrode lead 51 is attached to the positive electrode 53. Anegative-electrode lead 52 is attached to the negative electrode 54. Theoutermost portion of the spirally wound electrode 50 is protected by aprotective tape 57.

For example, the positive-electrode lead 51 and the negative-electrodelead 52 extend from the inside to the outside of the package members 59in one direction. The positive-electrode lead 51 is composed of aconductive material, e.g., Al. The negative-electrode lead 52 iscomposed of a conductive material, e.g., Cu, Ni, stainless steel. Thesematerials each have a sheet shape or a mesh shape.

For example, each of the package members 59 is formed of a laminatedfilm in which a bonding layer, a metal layer, and a surface protectivelayer are stacked in that order. For the laminated films, for example,peripheral portions of the bonding layers of two laminated films arebonded together by fusion bonding or with an adhesive in such a mannerthat the bonding layers face the spirally wound electrode 50. Each ofthe bonding layers is formed of a film of polyethylene, polypropylene,or the like. The metal layer is formed of Al foil or the like. Thesurface protective layer is formed of a film of nylon, polyethyleneterephthalate, or the like.

In particular, as each package member 59, an aluminum-laminated film inwhich a polyethylene film, aluminum foil, and a nylon film are stackedin that order is preferred. However, each package member 59 may beformed of a laminated film having another stacking structure.Alternatively, each package member 59 may be formed of a polymer film ofpolypropylene or a metal film.

Contact films 58 configured to prevent the entry of outside air arearranged between the positive-electrode lead 51 and the package members59 and between the negative-electrode lead 52 and the package members59. Each of the contact films 58 is composed of a material adhesive tothe positive-electrode lead 51 and the negative-electrode lead 52.Examples of the material include polyolefin resins, such aspolyethylene, polypropylene, modified polyethylene, and modifiedpolypropylene.

The positive electrode 53 includes, for example, a positive-electrodeactive material layer 53B provided on each surface of apositive-electrode collector 53A. The negative electrode 54 has the samestructure as the foregoing negative electrode for a lithium-ionsecondary battery. For example, the negative electrode 54 includes anegative-electrode active material layer 54B provided on each surface ofa negative-electrode collector 54A. The structures of thepositive-electrode collector 53A, the positive-electrode active materiallayers 53B, the negative-electrode collector 54A, and thenegative-electrode active material layers 54B are the same as those ofthe positive-electrode collector 21A, the positive-electrode activematerial layers 21B, the negative-electrode collector 22A, and thenegative-electrode active material layers 22B, respectively. Thestructure of each of the separators 55 is the same as that of theseparator 23.

Each of the electrolyte layers 56 is formed of a component in which anelectrolytic solution is held by a polymeric compound. Each electrolytelayer 56 may contain additional material, such as an additive, ifnecessary. The electrolyte layer 56 is composed of what is called agel-like electrolyte. The gel-like electrolyte is preferred because ahigh ionic conductivity (e.g., 1 mS/cm or more at room temperature) isobtained and the leakage of the electrolytic solution from the batteryis prevented.

The polymeric compound contains one or two or more compounds describedbelow. Examples thereof include polyacrylonitrile, polyvinylidenefluoride, polytetrafluoroethylene, polyhexafluoropropylene, polyethyleneoxide, polypropylene oxide, polyphosphazene, polysiloxane, polyvinylfluoride, polyvinyl acetate, polyvinyl alcohol, polymethyl methacrylate,polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber,nitrile-butadiene rubber, polystyrene, polycarbonate, and copolymers ofvinylidene fluoride and hexafluoropropylene. Among these compounds,polyvinylidene fluoride or copolymers of vinylidene fluoride andhexafluoropropylene are preferred because of their good electrochemicalstability.

For example, the electrolytic solution has the same composition as thecomposition of the electrolytic solution used in the prismatic secondarybattery. However, in the electrolyte layers 56 composed of a gel-likeelectrolyte, the solvent of the electrolytic solution indicates a wideconcept including not only a liquid solvent but also a material which iscapable of dissociating an electrolyte salt and which has ionicconductivity. Thus, in the case where a polymeric compound having ionicconductivity is used, the polymeric compound is included in the conceptof the solvent.

Instead of the gel-like electrolyte layers 56, an electrolytic solutionmay be used. In this case, the separators 55 are impregnated with theelectrolytic solution.

Operation of Secondary Battery

When the laminated-film-type secondary battery is charged, lithium ionsreleased from the positive electrode 53 are occluded in the negativeelectrode 54 through the electrolyte layers 56. When thelaminated-film-type secondary battery is discharged, lithium ionsreleased from the negative electrode 54 are occluded in the positiveelectrode 53 through the electrolyte layers 56.

Method for Producing Secondary Battery

The laminated-film-type secondary battery including the gel-likeelectrolyte layers 56 is produced by, for example, three types ofprocedures described below.

In a first procedure, first, the positive electrode 53 and the negativeelectrode 54 are formed in the same ways as the procedures for formingthe positive electrode 21 and the negative electrode 22. In this case,the positive-electrode active material layer 53B is formed on eachsurface of the positive-electrode collector 53A to form the positiveelectrode 53. The negative-electrode active material layer 54B is formedon each of the surfaces of the negative-electrode collector 54A to formthe negative electrode 54. Next, a precursor solution including anelectrolytic solution, a polymer compound, an organic solvent, and thelike is prepared. The precursor solution is applied to the positiveelectrode 53 and the negative electrode 54 to form the gel-likeelectrolyte layers 56. Then the positive-electrode lead 51 is attachedto the positive-electrode collector 53A by a welding method or the like.The negative-electrode lead 52 is attached to the negative-electrodecollector 54A by a welding method or the like. Subsequently, thepositive electrode 53 and the negative electrode 54 including theelectrolyte layers 56 are stacked with the separators 55 and spirallywound to form the spirally wound electrode 50. The protective tape 57 isbonded to the outermost portion of the spirally wound electrode 50.Finally, the spirally wound electrode 50 is sandwiched between twofilm-shaped package members 59. The peripheral portions of the packagemembers 59 are bonded together by a heat fusion method or the like toseal the spirally wound electrode 50 in the package members 59. In thiscase, the contact films 58 are interposed between the positive-electrodelead 51 and the package members 59 and between the negative-electrodelead 52 and the package members 59.

In a second procedure, first, the positive-electrode lead 51 is attachedto the positive electrode 53. The negative-electrode lead 52 is attachedto the negative electrode 54. Next, the positive electrode 53 and thenegative electrode 54 are stacked with the separators 55 providedtherebetween and spirally wound to form a spirally wound componentserving as a precursor of the spirally wound electrode 50. Theprotective tape 57 is bonded to the outermost portion of the spirallywound component. Then the spirally wound component is sandwiched betweentwo film-shaped package members 59. The peripheral portions except for aperipheral portion on one side are bonded by a heat fusion method or thelike to accommodate the spirally wound component in the pouch-likepackage members 59. Next, an electrolytic composition containing anelectrolytic solution, a monomer serving as a raw material of apolymeric compound, a polymerization initiator, and, optionally, anadditional material, such as a polymerization inhibitor, is prepared.The resulting electrolytic composition is injected into the pouch-likepackage members 59. The opening portion of the package members 59 issealed by a heat fusion method or the like. Finally, the monomer isthermally polymerized to form a polymeric compound, thereby resulting inthe gel-like electrolyte layers 56.

In a third procedure, first, a spirally wound component is formed andaccommodated in the pouch-like package members 59 in the same way as thesecond procedure, except that the separators 55 each having bothsurfaces coated with a polymeric compound is used. Examples of thepolymeric compound applied to the separators 55 include polymers fromvinylidene fluoride (homopolymers, copolymers, and multi-componentcopolymers). Specific examples thereof include polyvinylidene fluoride;two-component copolymers of vinylidene fluoride and hexafluoropropylene;and three-component copolymers of vinylidene fluoride,hexafluoropropylene, and chlorotrifluoroethylene. In combination with apolymer from vinylidene fluoride, one or two or more other polymericcompounds may be used. Next, the electrolytic solution is prepared andinjected into the package members 59. The opening portion of the packagemembers 59 is sealed by a heat fusion method or the like. Finally, thepackage members 59 are heated under load to bring the separators 55 intoclose contact with the positive electrode 53 and the negative electrode54 with the polymeric compound. Thus, the polymeric compound isimpregnated with the electrolytic solution. Thereby, the polymericcompound gels to form the electrolyte layers 56.

In the third procedure, swelling of the battery is suppressed, comparedwith the first procedure. A negligible amount of a monomer serving as araw material for the polymeric compound, an organic solvent, or the likeis left, compared with the second procedure, thereby satisfactorilycontrolling a step of forming the polymeric compound. Thus, theelectrolyte layers 56 have sufficient adhesion to the positive electrode53, the negative electrode 54, and the separators 55.

Function and Effect of Secondary Battery

For the laminated-film-type secondary battery, the negative electrode 54has the same structure as that of the foregoing negative electrode for alithium-ion secondary battery, thereby providing the same effects asthose of the prismatic secondary battery.

3. Application of Lithium-Ion Secondary Battery

Application examples of the foregoing lithium-ion secondary battery willbe described below.

The application of the lithium-ion secondary battery is not particularlylimited as long as the lithium-ion secondary battery is applied tomachines, devices, appliances, apparatuses, systems (combinations of aplurality of devices), and the like which can use the lithium-ionsecondary battery as a power source for operation or a power storagesource for accumulation of power. In the case where the lithium-ionsecondary battery is used as a power source, the power source may beused as a main power source (a power source to be preferentially used)or an auxiliary power source (a power source to be used instead of themain power source or by switching from the main power source). The typeof the main power source is not limited to the lithium-ion secondarybattery.

The lithium-ion secondary battery is applied to, for example, thefollowing applications. Examples of the applications include portableelectronic devices, such as video cameras, digital still cameras,cellular phones, notebook personal computers, cordless telephones,headphone stereos, portable radios, portable television sets, andpersonal digital assistants; portable home appliances, such as electricshavers; backup power sources; storage devices, such as memory cards;electric tools, such as electric drills and electric saws; battery packsused as power sources for notebook personal computers and so forth;medical electronic devices, such as pacemakers and hearing aids;vehicles, such as electric vehicles (including hybrid vehicles); andenergy storage systems, such as household battery systems storing powerin case of emergency or the like. The lithium-ion secondary battery maybe applied to applications other than the foregoing applications.

In particular, the lithium-ion secondary battery is effectively appliedto, for example, a battery pack, an electric vehicle, a power storagesystem, an electric tool, or an electronic device. This is because suchan application demands excellent battery characteristics; hence, the useof the lithium-ion secondary battery according to an embodiment of thepresent application effectively improves the characteristics. Thebattery pack refers to a power source including the lithium-ionsecondary batteries and is what is called a set of batteries or thelike. The electric vehicle refers to a vehicle that operates (runs)using the lithium-ion secondary battery as a power source for operation.As described above, the electric vehicle may include a vehicle (e.g., ahybrid vehicle) with a driving source in addition to the lithium-ionsecondary battery. The power storage system refers to a system includingthe lithium-ion secondary battery as a power storage source. Forexample, in a household energy storage system, electric power is storedin the lithium-ion secondary battery serving as a power storage source.Electric power is consumed when necessary, so home appliances can beused. The electric tool refers to a tool having a moving part (such as adrill) that is movable using the lithium-ion secondary battery as apower source for operation. The electronic device refers to a devicethat performs various functions using the lithium-ion secondary batteryfor operation.

Some application examples of the lithium-ion secondary battery will bespecifically described below. Configurations of the application examplesdescribed below are merely examples and thus can be appropriatelychanged.

3-1. Battery Pack

FIG. 10 illustrates a block configuration of a battery pack. Forexample, as illustrated in FIG. 10, the battery pack includes acontroller 61, a power source 62, a switching unit 63, a currentmeasurement unit 64, a temperature detecting unit 65, a voltagedetecting unit 66, a switching controller 67, memory 68, a temperaturedetecting element 69, a current detecting resistance 70, apositive-electrode terminal 71, and a negative-electrode terminal 72 ina housing 60 composed of, for example, a plastic material.

The controller 61 controls the overall operation of the battery pack(including the usage state of the power source 62) and includes, forexample, a central processing unit (CPU). The power source 62 includesone or two or more lithium-ion secondary batteries (not illustrated).The power source 62 refers to, for example, a set of batteries includingtwo or more lithium-ion secondary batteries. These lithium-ion secondarybatteries may be connected in series, in parallel, or in combinationthereof. For example, the power source 62 includes six lithium-ionsecondary batteries in which three sets of two batteries connected inparallel are connected in series.

The switching unit 63 is configured to switch the usage state of thepower source 62 (availability of connection between the power source 62and external equipment) in response to instructions from the controller61. The switching unit 63 includes, for example, a charge controlswitch, a discharge control switch, a diode for charge, and a diode fordischarge (all elements are not illustrated). Examples of the chargecontrol switch and the discharge control switch include semiconductorswitches formed of, for example, metal oxide semiconductor field-effecttransistors (MOSFETs).

The current measurement unit 64 is configured to measure a current withthe current detecting resistance 70 and to send the measurement resultsto the controller 61. The temperature detecting unit 65 is configured tomeasure a temperature with the temperature detecting element 69 and tosend the measurement results to the controller 61. For example, thetemperature measurement results are used when the controller 61 controlscharge and discharge at the time of abnormal heat generation and whenthe controller 61 performs correction at the time of the calculation ofremaining battery capacity. The voltage detecting unit 66 is configuredto measure the voltage of the lithium-ion secondary batteries in thepower source 62, subject the measured voltage to analog-to-digital (A/D)conversion, and send the resulting digital output to the controller 61.

The switching controller 67 is configured to control the operation ofthe switching unit 63 in response to signals from the currentmeasurement unit 64 and the voltage detecting unit 66.

The switching controller 67 controls the switching unit 63 in such amanner that, for example, when the battery voltage reaches an overchargedetection voltage, the switching unit 63 (charge control switch) isdisconnected so as not to allow a charging current to flow through thecurrent path of the power source 62. This permits the power source 62only to discharge with the diode for discharge. For example, theswitching controller 67 is configured to interrupt a charging currentwhen a large current flows during charging.

Furthermore, the switching controller 67 controls the switching unit 63in such a manner that, for example, when the battery voltage reaches anover-discharge detection voltage, the switching unit 63 (dischargecontrol switch) is disconnected so as not to allow a discharge currentto flow through the current path of the power source 62. This permitsthe power source 62 only to charge with the diode for charge. Forexample, the switching controller 67 is configured to interrupt adischarge current when a large current flows during discharging.

In the lithium-ion secondary battery, for example, the overchargedetection voltage is 4.20 V±0.05 V, and the over-discharge detectionvoltage is 2.4 V±0.1 V.

An example of the memory 68 is electrically erasable programmableread-only memory (EEPROM), which is nonvolatile memory. For example, thememory 68 stores a numerical value calculated by the controller 61 andinformation about the lithium-ion secondary batteries (for example,initial internal resistance) measured in the production process. In thecase where the full charge capacity of the lithium-ion secondarybatteries is stored in the memory 68, the controller 61 can obtaininformation about remaining capacity and so forth.

The temperature detecting element 69 is configured to measure thetemperature of the power source 62 and to send the measurement resultsto the controller 61. An example of the temperature detecting element 69is a thermistor.

The positive-electrode terminal 71 and the negative-electrode terminal72 are terminals for connection to external equipment (e.g., a notebookpersonal computer) operated by the battery pack or to external equipment(e.g., a charger) used to charge the battery pack. The charge anddischarge of the power source 62 are performed through thepositive-electrode terminal 71 and the negative-electrode terminal 72.

3-2. Electric Vehicle

FIG. 11 illustrates a block configuration of a hybrid vehicle as anexample of an electric vehicle. For example, as illustrated in FIG. 11,the electric vehicle includes a controller 74, an engine 75, a powersource 76, a driving motor 77, a differential device 78, a dynamo 79, atransmission 80, a clutch 81, inverters 82 and 83, and various sensors84 in a metal chassis 73. The electric vehicle further includes, forexample, an axle shaft 85 for front wheels, the axle shaft 85 beingconnected to the differential device 78 and the transmission 80, frontwheels 86, an axle shaft 87 for rear wheels, and rear wheels 88.

The electric vehicle can run using any one of the engine 75 and themotor 77 as a driving source. The engine 75 serves as a main drivingsource, such as a gasoline engine. In the case where the engine 75 isused as a driving source, for example, the driving force (torque) of theengine 75 is transmitted to the front wheels 86 or the rear wheels 88through the differential device 78, the transmission 80, and clutch 81serving as drive members. The torque of the engine 75 is alsotransmitted to the dynamo 79 and allows the dynamo 79 to generatealternating-current (AC) power. The AC power is converted intodirect-current (DC) power by the inverter 83. The resulting DC power isstored in the power source 76. Meanwhile, in the case where the motor 77serving as a convertor is used as a driving source, power (DC power)supplied from the power source 76 is converted into AC power by theinverter 82. The motor 77 is driven by the AC power. The driving force(torque) obtained by conversion of electric power using the motor 77 istransmitted to, for example, the front wheels 86 or the rear wheels 88through the differential device 78, the transmission 80, and clutch 81serving as drive members.

When the electric vehicle slows down with a brake mechanism (notillustrated), the resistance during the slowing down may be transmittedto the motor 77 in the form of torque, and the motor 77 may generate ACpower using the torque. Preferably, the resulting AC power is convertedinto DC power by the inverter 82, and the regenerative DC power isstored in the power source 76.

The controller 74 controls the operation of the overall electric vehicleand includes, for example, a central processing unit (CPU). The powersource 76 includes one or two or more lithium-ion secondary batteries(not illustrated). The power source 76 may be configured to be capableof storing power by establishing connection with an external powersource and receiving power from the external power source. For example,the various sensors 84 are used to control the number of revolutions ofthe engine 75 and to control the position (throttle position) of athrottle valve (not illustrated). The various sensors 84 include, forexample, a speed sensor, an acceleration sensor, and an engine speedsensor.

As described above, the hybrid vehicle has been described as an electricvehicle. The electric vehicle may be a vehicle (electric vehicle)operated by the power source 76 and the motor 77 without the engine 75.

3-3. Power Storage System

FIG. 12 illustrates a block configuration of a power storage system. Forexample, as illustrated in FIG. 12, the power storage system includes acontroller 90, a power source 91, a smart meter 92, and a power hub 93in a house 89, e.g., a general house or commercial building.

Here, for example, the power source 91 is connected to electricequipment 94 installed in the house 89 and can be connected to anelectric vehicle 96 parked outside the house 89. Furthermore, forexample, the power source 91 is connected to a private generator 95mounted on the house 89 via the power hub 93 and can be connected to anexternal centralized power grid 97 via the smart meter 92 and the powerhub 93.

The electric equipment 94 includes one or two or more home appliances,such as refrigerators, television sets, and water heaters. The privategenerator 95 includes one or two or more generators, such as solarphotovoltaic generators and wind generators. The electric vehicle 96includes one or two or more vehicles, such as electric vehicles,electric motorcycles, and hybrid vehicles. The centralized power grid 97includes one or two or more power grids connected to, for example,thermal power plants, nuclear power plants, hydroelectric powerstations, and wind farms.

The controller 90 controls the operation of the overall power storagesystem (including the usage state of the power source 91) and includes,for example, a CPU. The power source 91 includes one or two or morelithium-ion secondary batteries (not illustrated). The smart meter 92is, for example, a network-ready electrical meter installed in the house89, which is on the power demand side and can communicate with the powersupply side. Thus, for example, the smart meter 92 is configured tocontrol a balance between supply and demand in the house 89 whilecommunicating with the outside, if necessary, thereby efficiently andstably supplying energy.

In this power storage system, for example, power is stored in the powersource 91 from the centralized power grid 97, which is an external powersource, via the smart meter 92 and the power hub 93. Furthermore, poweris stored in the power source 91 from the private generator 95, which isan independent power source, via the power hub 93. The power stored inthe power source 91 is supplied to the electric equipment 94 or theelectric vehicle 96, if necessary, in response to instructions from thecontroller 90. Thus, the electric equipment 94 can be operated, and theelectric vehicle 96 can be charged. That is, the power storage system isa system capable of storing and supplying power in the house 89 usingthe power source 91.

Power stored in the power source 91 can be desirably used. For example,power can be stored in the power source 91 from the centralized powergrid 97 during late-night hours in which the market price of electricityis low. The power stored in the power source 91 can be used duringdaytime hours in which the market price of electricity is high.

The foregoing power storage system may be installed for each house(family) or may be installed for each set of a plurality of houses (aplurality of families).

3-4. Electric Tool

FIG. 13 illustrates a block configuration of an electric tool. Forexample, as illustrated in FIG. 13, the electric tool is an electricdrill that includes a controller 99 and a power source 100 in a mainbody 98 composed of, for example, a plastic material. For example, adrill portion 101, which is a moving part, is rotatably attached to themain body 98.

The controller 99 controls the overall operation of the electric tool(including the usage state of the power source 100) and includes, forexample, a CPU. The power source 100 includes one or two or morelithium-ion secondary batteries (not illustrated). The controller 99 isconfigured to appropriately supply power from the power source 100 tothe drill portion 101 in response to the operation of an operationswitch (not illustrated) to drive the drill portion 101.

EXAMPLES

Examples of the present application will be described in detail below.

Experimental Example 1-1 to 1-9

Laminated-film-type secondary batteries illustrated in FIGS. 8 and 9were produced by a procedure described below.

The positive electrode 53 was formed. First, 91 parts by mass of apositive-electrode active material (LiCoO₂), 6 parts by mass of apositive-electrode conductive agent (graphite), and 3 parts by mass of apositive-electrode binder (polyvinylidene fluoride: PVDF) were mixedtogether to form a positive-electrode mixture. The positive-electrodemixture was dispersed in an organic solvent (N-methyl-2-pyrrolidone:NMP) to form a paste-like slurry of the positive-electrode mixture. Thenthe slurry of the positive-electrode mixture was applied to both surfaceof the positive-electrode collector 53A with a coating apparatus anddried to form the positive-electrode active material layers 53B. As thepositive-electrode collector 53A, strip-shaped aluminum foil (thickness:12 μm) was used. Finally, the positive-electrode active material layers53B were subjected to compression forming with a roll press. Thethickness of each of the positive-electrode active material layers 53Bwas adjusted so as to prevent the deposition of metallic Li on thenegative electrode 54 in a fully charged state.

Next, the negative electrode 54 was formed. First, a core portion(SiO_(x)) was formed by a gas atomization method. A single-layercovering portion (SiO_(y)+M1 (Ni)) was formed on a surface of the coreportion by a powder evaporation method. The compositions of the coreportion and the covering portion (atomic ratios x and y, ratio(M1/(Si+O))) were described in Table 1. In this case, the core portionhad a half-width of 0.6°, a crystallite size of 90 nm, and a mediandiameter of 4 μm. The covering portion had an average thickness of 200nm and an average coverage of 70%.

When the core portion was formed, the oxygen flow rate was adjustedduring the melt-solidification of the raw material (Si) to control thecomposition (atomic ratio x). When the covering portion was formed,powdered SiO_(y) and powdered metal M1 were co-deposited. During thedeposition of the raw materials, the flow rate of O₂ or H₂ was adjustedto control the composition (atomic ratio y). Simultaneously, the inputpower was adjusted to control the ratio (M1/(Si+O)). In the powderevaporation method, a deflective electron beam evaporation source wasused. The raw material Si powder had a median diameter of 0.2 μm to 30μm. The deposition rate was 2 nm/sec. A vacuum state, i.e., a pressureof 1×10⁻³ Pa, was used with a turbo-molecular pump.

Next, a negative-electrode active material and a precursor of anegative-electrode binder were mixed together in a dry weight ratio of90:10. The resulting mixture was diluted with NMP to form a paste-likeslurry of a negative-electrode mixture. In this case, a polyamic acidcontaining NMP and N,N-dimethylacetamide (DMAC) was used. Then theslurry of the negative-electrode mixture was applied to both surface ofthe negative-electrode collector 54A with a coating apparatus and dried.As the negative-electrode collector 54A, rolled Cu foil (thickness: 15μm, ten-point height of irregularities Rz<0.5 μm) was used. Finally, inorder to enhance binding properties, the resulting coating films werehot-pressed and baked in a vacuum atmosphere at 400° C. for 1 hour.Thereby, the negative-electrode binder (polyamide-imide) was formed,thus resulting in the negative-electrode active material layers 54Bcontaining the negative-electrode active material and thenegative-electrode binder. The thickness of each of thenegative-electrode active material layers 54B was adjusted in such amanner that the negative-electrode utilization factor was 65%.

Next, an electrolyte salt (LiPF₆) was dissolved in a mixed solvent(ethylene carbonate (EC) and diethyl carbonate (DEC)) to prepare anelectrolytic solution. In this case, with respect to the composition ofthe mixed solvent, the ratio by weight of EC to DEC was 50 to 50, andthe proportion of the electrolyte salt was 1 mol/kg with respect to themixed solvent.

Finally, the secondary battery was assembled. First, thepositive-electrode lead 51 composed of Al was welded to an end of thepositive-electrode collector 53A. The negative-electrode lead 52composed of Ni was welded to an end of the negative-electrode collector54A. Next, the positive electrode 53, the separator 55, the negativeelectrode 54, and the separator 55 were stacked in that order. Theresulting stack was spirally wound in a longitudinal direction to form aspirally wound component serving as a precursor of the spirally woundelectrode 50. The outermost portion of the spirally wound component wasfixed with the protective tape 57 (adhesive tape). In this case, as eachof the separators 55, a laminated film (thickness: 20 μm) in which afilm mainly containing porous polyethylene was sandwiched between filmsmainly composed of porous polypropylene was used. Next, the spirallywound component was sandwiched between the package members 59. Theperipheral portions except for a peripheral portion on one side werebonded by heat fusion to accommodate the spirally wound component in thepouch-like package members 59. In this case, as each of the packagemembers 59, an aluminum-laminated film in which a nylon film (thickness:30 μm), Al foil (thickness: 40 μm), and a non-stretched polypropylenefilm (thickness: 30 μm) were stacked in that order from the outside wasused. Then the electrolytic solution was injected from an openingportion of the package members 59 to impregnate the separators 55 withthe electrolytic solution, thereby forming the spirally wound electrode50. Finally, the opening portion of the package members 59 was sealed byheat fusion in a vacuum atmosphere.

The cycle characteristics, the initial charge-discharge characteristics,and the load characteristics of the secondary batteries wereinvestigated. Table 1 illustrates the results.

In the case where the cycle characteristics were investigated, first,one charge-discharge cycle was performed in an atmosphere with atemperature of 23° C. in order to stabilize the battery state.Subsequently, another charge-discharge cycle was performed to measurethe discharge capacity. Next, the charge-discharge cycle was repeateduntil the number of cycles reached 100 cycles, and then the dischargecapacity was measured. Finally, the cycle retention rate was calculatedfrom the following expression: cycle retention rate (%)=(dischargecapacity at 100th cycle/discharge capacity at second cycle)×100. In thecase of charging, each secondary battery was charged at a constantcurrent density of 3 mA/cm² until the voltage reached 4.2 V, and thenthe battery was charged at a constant voltage of 4.2 V until the currentdensity reached 0.3 mA/cm². In the case of discharging, the battery wasdischarged at a constant current density of 3 mA/cm² until the voltagereached 2.5 V.

In the case where the initial charge-discharge characteristics wereinvestigated, first, one charge-discharge cycle was performed in orderto stabilize the battery state. Subsequently, each secondary battery wascharged again to measure the charge capacity. Then the battery wasdischarged to measure the discharge capacity. Finally, the initialefficiency was calculated from the following expression: initialefficiency (%)=(discharge capacity/charge capacity)×100. The atmospherictemperature and charge-discharge conditions were the same as those inthe case of investigating the cycle characteristics.

In the case of investigating the load characteristics, first, onecharge-discharge cycle was performed in order to stabilize the batterystate. Subsequently, the second cycle of the charge-discharge operationwas performed to measure the discharge capacity. Then the third cycle ofthe charge-discharge operation was performed to measure the dischargecapacity. Finally, the load retention rate was calculated from thefollowing expression: load retention rate (%)=(discharge capacity atthird cycle/discharge capacity at second cycle)×100. The atmospherictemperature and charge-discharge conditions were the same as those inthe case of investigating the cycle characteristics, except that thedischarge current density at the second cycle was changed to 0.2 mA/cm²and the discharge current density at the third cycle was changed to 1mA/cm².

TABLE 1 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 1-1 SiO_(x) 0.1SiO_(y) 1.2 Ni 1 80.1 80.2 94.0 1-2 5 82.2 82.2 96.0 1-3 10 84.0 83.697.0 1-4 20 85.0 84.0 97.5 1-5 30 84.0 83.2 98.0 1-6 50 83.5 82.5 98.01-7 60 82.0 81.5 98.0 1-8 SiO_(x) 0.1 — — — — 33.0 85.0 98.0 1-9 SiO_(x)0.1 SiO_(y) 1.2 — — 75.5 78.5 92.0

In the case where the covering portion (Si+O+Ni) was formed on thesurface of the core portion (Si+O), the cycle retention rate wassignificantly increased while maintaining high initial efficiency and ahigh load retention rate, as compared with the cases where the coveringportion was not formed and where the covering portion did not containNi.

Specifically, the formation of the covering portion (Si+O) on thesurface of the core portion (Si+O) resulted in a significant increase incycle retention rate but resulted in reductions in initial efficiencyand load retention rate, as compared with the case of the absence of thecovering portion. In contrast, the formation of the covering portion(Si+O+Ni) on the surface of the core portion (Si+O) resulted in afurther increase in cycle retention rate while maintaining an initialefficiency exceeding 80% and a load retention rate exceeding 90%, ascompared with the case of the absence of the covering portion. Theadvantageous tendency to a further increase in cycle retention ratewhile minimizing reductions in initial efficiency and load retentionrate is a specific tendency first accomplished by the formation of thecovering portion (Si+O+Ni).

In particular, in the case where the covering portion (Si+O+Ni) wasformed, an M1 ratio of 50 atomic percent or less resulted in inhibitionof a reduction in battery capacity, thus providing a high batterycapacity. In this case, an M1 ratio of 20 atomic percent or lessresulted in a higher battery capacity.

Experimental Example 2-1 to 2-94

As illustrated in Tables 2 to 7, secondary batteries were produced bythe same procedure as in Experimental Examples 1-1 to 1-7, except thatdifferent types and combinations of M1 metals were used. Characteristicsof each of the resulting secondary batteries were investigated. In thiscase, in order to perform co-deposition with powdered SiO_(y), eachpowdered metal M1 was used.

TABLE 2 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-1 SiO_(x) 0.1SiO_(y) 1.2 Al 1 80.0 80.1 94.0 2-2 10 83.2 82.6 96.0 2-3 20 83.6 82.997.0 2-4 50 81.5 81.0 98.0 2-5 60 81.0 80.2 98.0 2-6 SiO_(x) 0.1 SiO_(y)1.2 Fe 10 83.6 83.0 96.0 2-7 20 84.0 83.6 97.0 2-8 50 82.5 81.5 98.0 2-960 82.0 80.6 98.0

TABLE 3 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-10 SiO_(x) 0.1SiO_(y) 1.2 Cu 10 81.6 81.2 96.0 2-11 20 81.8 81.6 97.0 2-12 50 81.081.0 98.0 2-13 60 80.9 80.6 98.0 2-14 SiO_(x) 0.1 SiO_(y) 1.2 C 10 82.181.6 96.0 2-15 20 82.5 81.7 97.0 2-16 50 82.0 81.5 98.0 2-17 60 81.581.3 98.0 2-18 SiO_(x) 0.1 SiO_(y) 1.2 Mg 10 81.6 81.5 96.0 2-19 20 81.781.7 97.0 2-20 50 81.0 81.0 98.0 2-21 60 80.5 80.4 98.0 2-22 SiO_(x) 0.1SiO_(y) 1.2 Ca 10 82.1 81.0 96.0 2-23 20 82.5 81.5 97.0 2-24 50 82.081.0 98.0 2-25 60 81.6 80.9 98.0 2-26 SiO_(x) 0.1 SiO_(y) 1.2 Ti 10 82.581.5 96.0 2-27 20 83.0 82.0 97.0 2-28 50 82.6 82.0 98.0 2-29 60 82.081.6 98.0

TABLE 4 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-30 SiO_(x) 0.1SiO_(y) 1.2 Cr 10 82.0 81.0 96.0 2-31 20 82.5 81.5 97.0 2-32 50 81.681.2 98.0 2-33 60 81.5 80.6 98.0 2-34 SiO_(x) 0.1 SiO_(y) 1.2 Mn 10 81.581.0 96.0 2-35 20 81.9 81.5 97.0 2-36 50 81.4 81.6 98.0 2-37 60 81.281.0 98.0 2-38 SiO_(x) 0.1 SiO_(y) 1.2 Co 10 83.5 82.5 96.0 2-39 20 83.683.0 97.0 2-40 50 82.8 83.1 98.0 2-41 60 82.1 82.9 98.0 2-42 SiO_(x) 0.1SiO_(y) 1.2 Ge 10 83.0 82.0 97.0 2-43 20 83.1 82.2 97.0 2-44 50 83.182.1 98.0 2-45 60 83.0 82.1 98.0 2-46 SiO_(x) 0.1 SiO_(y) 1.2 Zr 10 82.181.5 96.0 2-47 20 82.5 81.6 97.0 2-48 50 82.1 81.5 98.0 2-49 60 81.681.3 98.0

TABLE 5 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-50 SiO_(x) 0.1SiO_(y) 1.2 Mo 10 82.1 82.0 97.0 2-51 20 82.6 82.1 97.0 2-52 50 81.182.2 98.0 2-53 60 80.5 82.1 98.0 2-54 SiO_(x) 0.1 SiO_(y) 1.2 Ag 10 80.681.5 96.0 2-55 20 80.9 81.7 96.0 2-56 50 80.5 81.5 97.0 2-57 60 80.381.4 98.0 2-58 SiO_(x) 0.1 SiO_(y) 1.2 Sn 10 82.2 82.5 97.0 2-59 20 82.382.4 97.0 2-60 50 82.3 82.5 98.0 2-61 60 82.2 82.5 98.0 2-62 SiO_(x) 0.1SiO_(y) 1.2 Ba 10 80.2 81.5 96.0 2-63 20 82.6 81.3 96.0 2-64 50 82.681.2 97.0 2-65 60 82.5 81.0 98.0 2-66 SiO_(x) 0.1 SiO_(y) 1.2 W 10 81.582.0 96.0 2-67 20 81.6 82.1 97.0 2-68 50 81.2 82.0 98.0 2-69 60 81.081.8 98.0

TABLE 6 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-70 SiO_(x) 0.1SiO_(y) 1.2 Ta 10 82.0 82.0 96.0 2-71 20 82.1 82.3 97.0 2-72 50 82.182.3 98.0 2-73 60 80.5 82.1 98.0 2-74 SiO_(x) 0.1 SiO_(y) 1.2 Na 10 81.581.5 96.0 2-75 20 81.8 81.6 96.0 2-76 50 81.0 81.5 97.0 2-77 60 81.081.3 98.0 2-78 SiO_(x) 0.1 SiO_(y) 1.2 K 10 81.2 81.3 96.0 2-79 20 81.681.5 96.0 2-80 50 81.2 81.4 97.0 2-81 60 81.0 81.2 98.0 2-82 SiO_(x) 0.1SiO_(y) 1.2 Li 1 82.5 80.3 97.0 2-83 5 82.4 83.6 98.0 2-84 10 82.4 85.598.0 2-85 20 82.1 87.5 98.0 2-86 40 82.1 90.2 98.0

TABLE 7 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 2-87 SiO_(x) 0.1SiO_(y) 1.2 Ni + Sn 5 + 5 83.2 84.0 97.0 2-88 10 + 10 83.3 84.2 97.02-89 25 + 25 83.4 84.1 98.0 2-90 30 + 30 83.0 84.1 98.0 2-91 SiO_(x) 0.1SiO_(y) 1.2 Ni + Li 5 + 5 83.0 85.0 97.0 2-92 10 + 5  83.1 85.5 98.02-93 25 + 10 83.1 87.5 98.0 2-94 30 + 10 83.0 88.0 98.0

Even when different types and combinations of M1 metals were used, highcycle retention rates, high initial efficiency, and high load retentionrates were obtained as with the results illustrated in Table 1.

Experimental Examples 3-1 to 3-7

As illustrated in Table 8, secondary batteries were produced by the sameprocedure as in Experimental Examples 1-1 to 1-7, except that thecovering portions had different compositions (different atomic ratiosy). Characteristics of each of the resulting secondary batteries wereinvestigated. In this case, the oxygen flow rate was adjusted during themelt-solidification of the raw material (Si) to control the atomic ratioy.

TABLE 8 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 3-1 SiO_(x) 0.1SiO_(y) 0.2 Ni 10 48.0 84.9 98.0 3-2 0.5 76.0 84.5 98.0 3-3 0.7 79.084.3 97.0 3-4 1 81.0 84.0 97.0 3-5 1.4 82.0 83.1 97.0 3-6 1.8 81.0 82.996.0 3-7 2 35.0 84.0 86.0

When the atomic ratio y was 0.5≦y≦1.5, a high cycle retention rate wasobtained.

Experimental Examples 4-1 to 4-9 and 5-1 to 5-10

As illustrated in Tables 9 and 10, secondary batteries were produced bythe same procedure as in Experimental Examples 1-1 to 1-7, except thatthe covering portions had different average coverage values and averagethickness values. Characteristics of each of the resulting secondarybatteries were investigated. In this case, during the formation of thecovering portion, the input power and the deposition time were changedto control the average coverage, and the deposition rate and thedeposition time were changed to control the average thickness.

TABLE 9 Cycle Load Covering portion Average retention Initial retentionExperimental Core portion Proportion coverage rate efficiency rateexample Composition x Composition y M1 (at. %) (%) (%) (%) (%) 4-1SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 10 76.0 86.0 95.0 4-2 20 78.0 85.6 95.04-3 30 81.0 85.2 96.0 4-4 40 82.0 84.5 96.0 4-5 50 83.0 84.2 96.0 4-6 6083.5 84.0 96.0 4-7 80 84.5 83.0 97.0 4-8 90 85.0 82.5 97.0 4-9 100 85.082.0 98.0

TABLE 10 Cycle Load Covering portion Average retention Initial retentionExperimental Core portion Proportion thickness rate efficiency rateexample Composition x Composition y M1 (at. %) (nm) (%) (%) (%) 5-1SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 1 75.6 84.5 95.0 5-2 10 78.0 84.0 95.0 5-3100 82.0 83.8 96.0 5-4 500 85.0 83.4 97.0 5-5 1000 85.5 82.5 98.0 5-62000 85.6 82.0 97.0 5-7 3000 85.6 81.5 97.0 5-8 5000 85.6 80.7 96.0 5-910000 85.7 80.2 95.0 5-10 15000 85.7 79.0 95.0

In the case where the average coverage was 30% or more and where theaverage thickness was in the range of 1 nm to 10,000 nm, a high cycleretention rate was obtained.

Experimental Examples 6-1 to 6-5

As illustrated in Table 11, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except that thecovering portions had different layer structures. Characteristics ofeach of the resulting secondary batteries were investigated. In thiscase, a process for forming the covering portion was performed in twodivided steps, thereby providing the multilayer covering portion.Furthermore, when the covering portion was formed, the substratetemperature during codeposition was changed, thereby controlling thebonding state in the covering portion.

TABLE 11 Covering portion Cycle Initial Load Experimental Core portionProportion Layer Bonding retention efficiency retention exampleComposition x Composition y M1 (at. %) structure state rate (%) (%) rate(%) 6-1 SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 multilayer SiO + Ni 84.0 83.5 97.06-2 single SiNiO + Ni 84.0 83.6 98.0 layer 6-3 single SiO + Ni 84.1 83.598.0 layer 6-4 multilayer SiNiO + Ni 84.1 83.5 98.0 6-5 multilayerSiNiO + Ni/ 84.0 83.6 98.0 SiO + Ni

The presence of the multilayer covering portion increased the cycleretention rate. Furthermore, the presence of SiNiO in the coveringportion further increased the cycle retention rate.

Experimental Examples 7-1 to 7-5

As illustrated in Table 12, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except that thecore portions had different compositions (atomic ratios x).Characteristics of each of the resulting secondary batteries wereinvestigated. In this case, the oxygen flow rate was adjusted during themelt-solidification of the raw material (Si) to control the atomic ratiox.

TABLE 12 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition x Composition y M1 (at. %) (%) (%) (%) 7-1 SiO_(x) 0 SiO_(y)1.2 Ni 10 83.0 84.5 97.0 7-2 0.05 83.5 84.0 97.0 7-3 0.3 84.2 82.0 97.07-4 0.5 84.5 80.0 97.0 7-5 0.7 84.9 78.7 96.0

When the atomic ratio x was 0≦x<0.5, the cycle retention rate and theinitial efficiency were further increased.

Experimental Examples 8-1 to 8-3

As illustrated in Table 13, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except that thecore portions were composed of different materials. Characteristics ofeach of the resulting secondary batteries were investigated. As thematerial constituting the core portion, a Sn alloy was used.

TABLE 13 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate exampleComposition Composition y M1 (at. %) (%) (%) (%) 8-1 SnCo SiO_(y) 1.2 Ni10 83.2 82.0 95.0 8-2 SnCoTi 83.4 82.1 96.0 8-3 SnFeCo 83.4 82.0 96.0

Even when the core portion was composed of elemental Sn or the Sn alloy,a high cycle retention rate, high initial efficiency, and a high loadretention rate were obtained.

Experimental Examples 9-1 to 9-14

As illustrated in Table 14, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except thatelement M2 (e.g., Fe) was incorporated into each core portion.Characteristics of each of the resulting secondary batteries wereinvestigated. In this case, the core portion was formed by a gasatomization method using powdered SiO_(x) and powdered metal M2 (e.g.,Al) as raw materials.

TABLE 14 Cycle Load Core portion Covering portion retention Initialretention Experimental Proportion Proportion rate efficiency rateexample Composition x M2 (at. %) Composition y M1 (at. %) (%) (%) (%)9-1 SiO_(x) 0.1 Al 0.01 SiO_(y) 1.2 Ni 10 84.2 83.6 97.0 9-2 0.1 84.383.5 97.0 9-3 1 84.6 83.5 98.0 9-4 10 85.0 83.5 98.0 9-5 30 85.2 83.498.0 9-6 50 85.6 83.4 98.0 9-7 60 85.6 83.3 98.0 9-8 SiO_(x) 0.1 Fe 0.01SiO_(y) 1.2 Ni 10 84.5 83.8 97.0 9-9 0.1 84.6 83.7 97.0 9-10 1 84.8 83.598.0 9-11 10 85.0 83.5 98.0 9-12 30 85.3 84.0 98.0 9-13 50 85.3 84.098.0 9-14 60 85.4 83.6 98.0

The incorporation of M2 into the core portion resulted in a furtherincrease in cycle retention rate. In this case, an M2 ratio of 0.01atomic percent to 50 atomic percent resulted in a higher batterycapacity.

Experimental Examples 10-1 to 10-60

As illustrated in Tables 15 to 17, secondary batteries were produced bythe same procedure as in Experimental Examples 1-1 to 1-7, except thatelement M3 (e.g., Cr) or element M4 (e.g., B) was incorporated into eachcore portion. Characteristics of each of the resulting secondarybatteries were investigated. In this case, the core portion was formedby a gas atomization method using powdered SiO_(x) and, for example,powdered metal M3 (Cr or the like) as raw materials.

TABLE 15 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate example (M3,M4) Composition y M1 (at. %) (%) (%) (%) 10-1 Si₅₀Al₄₉Cr₁ SiO_(y) 1.2 Ni10 86.1 83.8 98.0 10-2 Si₅₀Al₄₉Ni₁ 86.0 83.7 97.0 10-3 Si₅₀Al₄₉Fe₁ 86.283.5 98.0 10-4 Si₄₀Al₄₁Cr₁₉ 86.3 83.5 97.0 10-5 Si₄₀Al₄₁Ni₁₉ 86.2 83.698.0 10-6 Si₄₀Al₄₁Fe₁₉ 86.4 83.5 97.0 10-7 Si₃₅Al₄₆Cr₁₉ 86.3 83.5 97.010-8 Si₃₅Al₄₆Ni₁₉ 86.2 83.5 97.0 10-9 Si₃₅Al₄₆Fe₁₉ 86.2 83.4 97.0 10-10Si₃₀Al₂₀Cr₅₀ 86.1 83.4 97.0 10-11 Si₃₀Al₂₀Ni₅₀ 86.4 83.3 97.0 10-12Si₃₀Al₂₀Fe₅₀ 86.2 83.5 98.0 10-13 Si₃₀Al₁₀Cr₆₀ 86.1 83.6 97.0 10-14Si₃₀Al₁₀Ni₆₀ 86.0 83.5 98.0 10-15 Si₃₀Al₁₀Fe₆₀ 86.2 83.5 97.0 10-16Si₃₀Al_(47.5)Cr_(22.49)Cu_(0.01) 86.3 83.5 97.0 10-17Si₃₀Al_(47.5)Ni_(22.49)Cu_(0.01) 86.2 83.7 98.0 10-18Si₃₀Al_(47.5)Fe_(22.49)Cu_(0.01) 86.4 83.5 97.0 10-19Si₃₀Al_(47.5)Cr_(12.5)Cu₁₀ 86.3 83.5 97.0 10-20Si₃₀Al_(47.5)Ni_(12.5)Cu₁₀ 86.2 83.6 97.0 10-21Si₃₀Al_(47.5)Fe_(12.5)Cu₁₀ 86.2 83.5 97.0 10-22 Si₃₀Al₂₅Cr₂₅Cu₂₀ 86.283.5 97.0 10-23 Si₃₀Al₂₅Ni₂₅Cu₂₀ 86.1 83.5 97.0 10-24 Si₃₀Al₂₅Fe₂₅Cu₂₀86.4 83.4 98.0 10-25 Si₃₀Al₂₀Cr₃₀Cu₂₀ 86.2 83.4 97.0 10-26Si₃₀Al₂₀Ni₃₀Cu₂₀ 86.1 83.3 98.0 10-27 Si₃₀Al₂₀Fe₃₀Cu₂₀ 86.0 83.5 97.0

TABLE 16 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate example (M3,M4) Composition y M1 (at. %) (%) (%) (%) 10-28Si₃₀Al_(27.5)Cr_(12.5)Cu₃₀ SiO_(y) 1.2 Ni 10 86.2 83.5 97.0 10-29Si₃₀Al_(27.5)Ni_(12.5)Cu₃₀ 86.3 83.4 98.0 10-30Si₃₀Al_(27.5)Fe_(12.5)Cu₃₀ 86.2 83.4 97.0 10-31Si₃₀Al₂₀Cr_(12.5)Cu_(37.5) 86.3 83.3 97.0 10-32Si₃₀Al₂₀Ni_(12.5)Cu_(37.5) 86.2 83.5 97.0 10-33Si₃₀Al₂₀Fe_(12.5)Cu_(37.5) 86.4 83.6 97.0 10-34Si₃₀Al_(47.5)Cr_(12.5)B₁₀ 86.3 83.5 97.0 10-35Si₃₀Al_(47.5)Cr_(12.5)Mg₁₀ 86.2 83.5 97.0 10-36Si₃₀Al_(47.5)Cr_(12.5)Ca₁₀ 86.2 83.5 97.0 10-37Si₃₀Al_(47.5)Cr_(12.5)Ti₁₀ 86.2 83.7 97.0 10-38Si₃₀Al_(47.5)Cr_(12.5)V₁₀ 86.4 83.5 98.0 10-39Si₃₀Al_(47.5)Cr_(12.5)Mn₁₀ 86.2 83.5 97.0 10-40Si₃₀Al_(47.5)Cr_(12.5)Co₁₀ 86.1 83.6 98.0 10-41Si₃₀Al_(47.5)Cr_(12.5)Ge₁₀ 86.0 83.5 97.0 10-42Si₃₀Al_(47.5)Cr_(12.5)Y₁₀ 86.2 83.5 97.0 10-43Si₃₀Al_(47.5)Cr_(12.5)Zr₁₀ 86.3 83.4 98.0 10-44Si₃₀Al_(47.5)Cr_(12.5)Mo₁₀ 86.2 83.4 97.0 10-45Si₃₀Al_(47.5)Cr_(12.5)Ag₁₀ 86.3 83.3 97.0 10-46Si₃₀Al_(47.5)Cr_(12.5)In₁₀ 86.4 83.5 97.0 10-47Si₃₀Al_(47.5)Cr_(12.5)Sn₁₀ 86.3 83.5 98.0 10-48Si₃₀Al_(47.5)Cr_(12.5)Sb₁₀ 86.2 83.4 97.0 10-49Si₃₀Al_(47.5)Cr_(12.5)Ta₁₀ 86.2 83.4 98.0 10-50Si₃₀Al_(47.5)Cr_(12.5)W₁₀ 86.2 83.3 97.0 10-51Si₃₀Al_(47.5)Cr_(12.5)Pb₁₀ 86.4 83.5 97.0 10-52Si₃₀Al_(47.5)Cr_(12.5)La₁₀ 86.2 83.5 98.0 10-53Si₃₀Al_(47.5)Cr_(12.5)Ce₁₀ 86.1 83.5 97.0 10-54Si₃₀Al_(47.5)Cr_(12.5)Pr₁₀ 86.4 83.7 98.0 10-55Si₃₀Al_(47.5)Cr_(12.5)Nd₁₀ 86.2 83.5 97.0

TABLE 17 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion rate efficiency rate example (M3,M4) Composition y M1 (at. %) (%) (%) (%) 10-56Si₁₀Al_(47.5)Cr_(12.5)Cu₃₀ SiO_(y) 1.2 Ni 10 86.4 83.5 98.0 10-57Si₂₀Al_(47.5)Cr_(7.5)Cu₅ 86.2 83.6 97.0 10-58 Si₈₀Al₁₀Cr₅Cu₅ 86.1 83.597.0 10-59 Si₈₅Al₅Cr₅Cu₅ 86.0 83.3 98.0 10-60 Si₃₀Al_(47.5)Cr_(12.5)Cu₁₀86.2 83.3 97.0

The incorporation of M3 or M4 into the core portion resulted in afurther increase in cycle retention rate. In this case, when the M3ratio was in the range of 1 atomic percent to 50 atomic percent and whenthe M4 ratio was in the range of 0.01 atomic percent to 30 atomicpercent, a higher battery capacity was obtained.

Experimental Examples 11-1 to 11-8

As illustrated in Table 18, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except that aconductive portion was formed on the surface of each core portion.Characteristics of each of the resulting secondary batteries wereinvestigated. In this case, each conductive portion was formed by thesame procedure as the procedure for forming the covering portion. Theaverage thickness and the average coverage of each conductive portionare described in Table 18.

TABLE 18 Conductive portion Cycle Load Covering portion Average Averageretention Initial retention Experimental Core portion Proportionthickness coverage rate efficiency rate example Composition xComposition y M1 (at. %) Type (nm) (%) (%) (%) (%) 11-1 SiO_(x) 0.1SiO_(y) 1.2 Ni 10 C 100 5 84.0 83.6 97.0 11-2 10 84.0 83.6 98.0 11-3 1584.1 83.6 98.0 11-4 30 84.1 83.7 98.0 11-5 50 84.1 83.7 99.0 11-6 7084.1 83.7 99.0 11-7 90 84.1 83.8 99.0 11-8 99 84.2 83.8 99.0

The formation of the conductive portion provided better results.

Experimental Examples 12-1 to 12-6

As illustrated in Table 19, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except that thecore portions had different median diameters. Characteristics of each ofthe resulting secondary batteries were investigated.

TABLE 19 Core portion Cycle Load Median Covering portion retentionInitial retention Experimental diameter Proportion rate efficiency rateexample Composition x (nm) Composition y M1 (at. %) (%) (%) (%) 12-1SiO_(x) 0.1 0.1 SiO_(y) 1.2 Ni 10 82.6 79.2 97.0 12-2 0.3 83.0 80.5 97.012-3 1 83.3 82.3 97.0 12-4 10 82.0 84.5 98.0 12-5 20 81.0 83.0 98.0 12-630 76.0 79.0 96.0

A median diameter of the core portion of 0.3 μm to 20 μm resulted in ahigh cycle retention rate, high initial efficiency, and a high batterycapacity.

Experimental Examples 13-1 to 13-18

As illustrated in Table 20, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except thatdifferent negative-electrode binders were used. Characteristics of eachof the resulting secondary batteries were investigated. In this case, asthe negative-electrode binders, polyimide (PI), polyvinylidene fluoride(PVDF), polyamide (PA), polyacrylic acid (PAA), lithium polyacrylate(PAAL), and carbonized polyimide (carbonized PI) were used. In the casewhere PAA or PAAL was used, a slurry of a negative-electrode mixture wasprepared with an aqueous solution containing 17% by volume PAA or PAAL.Hot-pressing was performed to form the negative-electrode activematerial layers 54B without baking.

TABLE 20 Cycle Load Covering portion Negative- retention Initialretention Experimental Core portion Proportion electrode rate efficiencyrate example Type x Type y M1 (at. %) binder (%) (%) (%) 13-1 SiO_(x)0.1 SiO_(y) 1.2 Ni 10 PI 83.5 83.4 97.0 13-2 20 84.6 83.5 97.0 13-3 5083.3 82.1 97.0 13-4 SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 PVDF 82.8 83.1 97.013-5 20 84.0 83.3 97.0 13-6 50 83.0 83.0 97.0 13-7 SiO_(x) 0.1 SiO_(y)1.2 Ni 10 PA 83.0 83.0 97.0 13-8 20 84.0 83.5 97.0 13-9 50 83.0 83.197.0 13-10 SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 PAA 82.6 83.1 97.0 13-11 2083.1 83.3 97.0 13-12 50 82.8 83.3 97.0 13-13 SiO_(x) 0.1 SiO_(y) 1.2 Ni10 PAAL 83.5 84.1 97.0 13-14 20 84.2 84.2 97.0 13-15 50 83.5 84.1 97.013-16 SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 carbonized PI 83.6 84.5 97.0 13-1720 84.5 84.6 97.0 13-18 50 83.9 84.4 97.0

Even when different negative-electrode binders were used, high cycleretention rates, high initial efficiency, and high load retention rateswere obtained.

Experimental Examples 14-1 to 14-12

As illustrated in Table 21, secondary batteries were produced by thesame procedure as in Experimental Examples 1-1 to 1-7, except thatdifferent positive-electrode active materials were used. Characteristicsof each of the resulting secondary batteries were investigated.

TABLE 21 Cycle Load Covering portion retention Initial retentionExperimental Core portion Proportion Positive electrode active rateefficiency rate example Type x Type y M1 (at. %) material (%) (%) (%)14-1 SiO_(x) 0.1 SiO_(y) 1.2 Ni 10 LiNi_(0.70)Co_(0.25)Al_(0.05)O₂ 84.183.5 97.0 14-2 LiNi_(0.79)Co_(0.14)Al_(0.07)O₂ 84.0 83.6 97.0 14-3LiNi_(0.70)Co_(0.25)Mg_(0.05)O₂ 84.2 83.7 97.0 14-4LiNi_(0.70)Co_(0.25)Fe_(0.05)O₂ 84.1 83.6 97.0 14-5 LiNiO₂ 84.1 83.797.0 14-6 LiNi_(0.33)Co_(0.33)Mn_(0.33)O₂ 84.2 83.6 97.0 14-7LiNi_(0.13)Co_(0.60)Mn_(0.27)O₂ 84.1 83.7 97.0 14-8Li_(1.13)[Ni_(0.22)Co_(0.18)Mn_(0.60)]_(0.87)O₂ 84.2 83.5 97.0 14-9Li_(1.13)[Ni_(0.20)Co_(0.20)Mn_(0.60)]_(0.87)O₂ 84.0 83.6 97.0 14-10Li_(1.13)[Ni_(0.18)Co_(0.22)Mn_(0.60)]_(0.87)O₂ 84.2 83.6 97.0 14-11Li_(1.13)[Ni_(0.25)Co_(0.25)Mn_(0.50)]_(0.87)O₂ 84.2 83.5 97.0 14-12Li₂Ni_(0.40)Cu_(0.60)O₂ 84.1 83.7 97.0

Even when different positive-electrode active materials were used, highcycle retention rates, high initial efficiency, and high load retentionrates were obtained.

The results illustrated in Tables 1 to 21 demonstrated that when thenegative-electrode active material included the covering portion havinga predetermined composition on the surface of the core portion, highcycle characteristics, high initial charge-discharge characteristics,and high load characteristics were obtained.

While the present application has been described above with reference tothe embodiments and examples, the present application is not limited tothese embodiments and examples. Various modifications may be made. Forexample, while the capacity of the negative electrode has beenrepresented on the basis of the occlusion and release of lithium ions inthe above-described embodiments, the present application is notnecessarily limited thereto. The present application is also applicableto the case where the capacity of a negative electrode includes acapacity on the basis of the occlusion and release of lithium ions and acapacity on the basis of the deposition and dissolution of metallic Li,and is expressed as the sum of the capacities. In this case, as thenegative-electrode active material, a negative-electrode materialcapable of occluding and releasing lithium ions is used, and thechargeable capacity of the negative-electrode material is set to behigher than the discharge capacity of the positive electrode.

While the case where the battery has a prismatic shape, a cylindricalshape, or a laminated-film shape and where the battery element has aspirally wound structure has been described above, the presentapplication is not necessarily limited thereto. The present applicationis also applicable to a battery having a prismatic shape, a buttonshape, or the like, or to a battery element having a laminatedstructure.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention is claimed as follows:
 1. A battery comprising: a positiveelectrode; a negative electrode including a first negative electrodeactive material; and an electrolytic solution, wherein the firstnegative electrode active material includes: a core portion having acore portion surface, wherein the core portion has a median diameter of0.3 μm to 20 μm, and a covering portion that covers at least part of thecore portion surface, wherein the covering portion comprises at leastSi, O and at least of one element M1 selected from Li, carbon (C), Mg,Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na,and K.
 2. The battery according to claim 1, wherein the covering portioncovers from 30% to 100% of the core portion surface.
 3. The batteryaccording to claim 1, wherein the covering portion has a thickness of 1nm to 10,000 nm.
 4. The battery according to claim 1, wherein thecovering portion has a multilayer structure.
 5. The battery according toclaim 1, wherein the first negative electrode active material includes aconductive portion that covers at least part of a surface of thecovering portion, and wherein the conductive portion has a lowerelectrical resistance than any one of the core portion and the coveringportion.
 6. The battery according to claim 1, wherein the negativeelectrode further includes a second negative electrode active material.7. The battery according to claim 6, wherein the second negativeelectrode active material includes a carbon material.
 8. The batteryaccording to claim 7, wherein the core portion includes carbon.
 9. Thebattery according to claim 8, wherein the element M1 is carbon.
 10. Thebattery according to claim 9, wherein the positive electrode includeslithium cobalt oxide.
 11. The battery according to claim 1, wherein anatomic ratio y (O/Si) of O to Si in the covering portion is 0.5≦y≦1.8.12. The battery according to claim 1, wherein the core portion includesat least one of Si and Sn.
 13. The battery according to claim 7, whereinthe electrolytic solution includes a nonaqueous solvent comprising atleast one selected from the group consisting of a halogenated chaincarbonate and a halogenated cyclic carbonate.
 14. The battery accordingto claim 13, wherein the halogenated cyclic carbonate comprises at leastone selected from the group consisting of 4-fluoro-1,3-dioxolan-2-oneand 4,5-difluoro-1,3-dioxolan-2-one.
 15. The battery according to claim13, wherein an amount of the at least one selected from the groupconsisting of the halogenated chain carbonate and the halogenated cycliccarbonate ranges from 0.01% by weight to 50% by weight of the nonaqueoussolvent.
 16. The battery according to claim 7, wherein the negativeelectrode includes a negative electrode collector comprising at leastone selected from the group consisting of carbon and sulfur.
 17. Thebattery according to claim 16, wherein a content of the at least oneselected from the group consisting of carbon and sulfur in the negativeelectrode collector is 100 ppm or less.
 18. An electric tool comprising:a battery including a positive electrode, a negative electrode includinga negative electrode active material, and an electrolytic solution; anda moving part configured to be powered by the battery, wherein thenegative electrode active material includes: a core portion having acore portion surface, wherein the core portion has a median diameter of0.3 μm to 20 μm, and a covering portion that covers at least part of thecore portion surface, wherein the covering portion comprises at leastSi, O and at least of one element M1 selected from Li, carbon (C), Mg,Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na,and K.
 19. A negative electrode active material comprising: a coreportion having a core portion surface, wherein the core portion has amedian diameter of 0.3 μm to 20 μm, and a covering portion that coversat least a part of the core portion surface, wherein the coveringportion comprises at least Si and O and at least of one element M1selected from Li, carbon (C), Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu,Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na, and K.
 20. The negative electrodeactive material according to claim 19, wherein the covering portioncovers from 30% to 100% of the core portion surface.
 21. The negativeelectrode active material according to claim 19, wherein the coveringportion has a thickness of 1 nm to 10,000 nm.
 22. The negative electrodeactive material according to claim 19, wherein the covering portion hasa multilayer structure.
 23. The negative electrode active materialaccording to claim 19, further comprising a conductive portion thatcovers at least part of a surface of the covering portion, wherein theconductive portion has a lower electrical resistance than any one of thecore portion and the covering portion.
 24. The negative electrode activematerial according to claim 19, wherein the element M1 is carbon. 25.The negative electrode active material according to claim 19, wherein anatomic ratio y (O/Si) of O to Si is 0.5≦y≦1.8.
 26. The negativeelectrode active material according to claim 19, wherein the coreportion includes at least one of Si and Sn.
 27. The negative electrodeactive material according to claim 19, wherein the core portion includescarbon.